 • Search in book...
• Toggle Font Controls

## Chapter 9Calculation groups

In 2019, DAX received a major update with the introduction of calculation groups. Calculation groups are a utility feature inspired from a similar feature available in MDX, known as calculated members. If you already know what calculated members in MDX are, then learning calculation groups should be somewhat easier. However, the DAX implementation differs from the MDX implementation. Therefore, regardless of your previous knowledge, in this chapter you will learn what calculation groups are, what they were designed for, and how they can help build awesome calculations.

Calculation groups are easy to use; however, designing a model with calculation groups correctly can be challenging when you create multiple calculation groups or when you use calculation items in measures. For this reason, we provide best practices to help you avoid any issues. Deviating from these best practices requires a deep understanding of how calculation groups are designed, if one wants to obtain a sound model.

Calculation groups are a new feature in DAX, which, as of April 2019, has not been completed and released. Along this chapter we highlight the parts that may change in the final version of this feature. Therefore, it is important to visit the web page https://www.sqlbi.com/calculation-groups, where you will find updated material and examples about calculation groups in DAX.

### Introducing calculation groups

Before we provide a description of calculation groups, it is useful to spend some time analyzing the business requirement that led to the introduction of this feature. Because you just finished digesting the chapter about time intelligence, an example involving time-related calculations fits perfectly well.

In our sample model we defined calculations to compute the sales amount, the total cost, the margin, and the total quantity sold by using the following DAX code:

```Sales Amount := SUMX ( Sales, Sales[Quantity] * Sales[Net Price] )
Total Cost := SUMX ( Sales, Sales[Quantity] * Sales[Unit Cost] )
Margin := [Sales Amount] - [Total Cost]
Sales Quantity := SUM ( Sales[Quantity] )```

All four measures are useful, and they provide different insights into the business. Moreover, all four measures are good candidates for time intelligence calculations. A year-to-date over sales quantity can be as interesting as a year-to-date over sales amount and over margin. The same consideration is true for many other time intelligence calculations: same period last year, growth in percentage against the previous year, and many others.

Nevertheless, if one wants to build all the different time intelligence calculations for all the measures, the number of measures in the data model may grow very quickly. In the real world, managing a data model with hundreds of measures is intimidating for both users and developers. Finally, consider that all the different measures for time intelligence calculations are simple variations of a common pattern. For example, the year-to-date versions of the previous list of four measures would look like the following:

```YTD Sales Amount :=
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
)

YTD Total Cost :=
CALCULATE (
[Total Cost],
DATESYTD ( 'Date'[Date] )
)

YTD Margin :=
CALCULATE (
[Margin],
DATESYTD ( 'Date'[Date] )
)

YTD Sales Quantity :=
CALCULATE (
[Sales Quantity],
DATESYTD ( 'Date'[Date] )
)```

All the previous measures only differ in their base measure; they all apply the same DATESYTD filter context to different base measures. It would be great if a developer were given the opportunity to define a more generic calculation, using a placeholder for the measure:

```YTD <Measure> :=
CALCULATE (
<Measure>,
DATESYTD ( 'Date'[Date] )
)```

The previous code is not a valid DAX syntax, but it provides a very good description of what calculation items are. You can read the previous code as: When you need to apply the YTD calculation to a measure, call the measure after applying DATESYTD to the Date[Date] column. This is what a calculation item is: A calculation item is a DAX expression containing a special placeholder. The placeholder is replaced with a measure by the engine just before evaluating the result. In other words, a calculation item is a variation of an expression that can be applied to any measure.

Moreover, a developer will likely find themselves needing several time intelligence calculations. As we noted at the beginning of the section, year-to-date, quarter-to-date, and same period last year are all calculations that somehow belong to the same group of calculations. Therefore, DAX offers calculation items and calculation groups. A calculation group is a set of calculation items that are conveniently grouped together because they are variations on the same topic.

Let us continue with DAX pseudo-code:

```CALCULATION GROUP "Time Intelligence"
CALCULATION ITEM CY := <Measure>
CALCULATION ITEM PY := CALCULATE ( <Measure>, SAMPEPERIODLASTYEAR ( 'Date'[Date] ) )
CALCULATION ITEM QTD := CALCULATE ( <Measure>, DATESQTD ( 'Date'[Date] ) )
CALCULATION ITEM YTD := CALCULATE ( <Measure>, DATESYTD ( 'Date'[Date] ) )```

As you can see, we grouped four time-related calculations in a group named Time Intelligence. In only four lines, the code defines dozens of different measures because the calculation items apply their variation to any measure in the model. Thus, as soon as a developer creates a new measure, the CY, PY, QTD, and YTD variations will be available at no cost.

There are still several details missing in our understanding of calculation groups, but only one is required to start taking advantage of them and to define the first calculation group: How does the user choose one variation? As we said, a calculation item is not a measure; it is a variation of a measure. Therefore, a user needs a way to put in a report a specific measure with one or more variations of the measure itself. Because users have the habit of selecting columns from tables, calculation groups are implemented as if they were columns in tables, whereas calculation items are like values of the given columns. This way, the user can use the calculation group in the columns of a matrix to display different variations of a measure in the report. For example, the calculation items previously described are applied to the columns of the matrix in Figure 9-1, showing different variations of the Sales Amount measure. Figure 9-1 The user can use a calculation group as if it were a column of the model, applying it to matrix columns.

### Creating calculation groups

The implementation of calculation groups in a Tabular model depends on the user interface of the editor tool. At the time of writing (April 2019), neither Power BI nor SQL Server Data Tools (SSDT) for Analysis Services have a user interface for this feature, which is only available at the API level of Tabular Object Model (TOM). The first tool providing an editor for this feature is Tabular Editor, an open source tool available for free at https://tabulareditor.github.io/.

In Tabular Editor, the Model / New Calculation Group menu item creates a new calculation group, which appears as a table in the model with a special icon. This is shown in Figure 9-2 where the calculation group has been renamed Time Intelligence.

A calculation group is a special table with a single column, named Attribute by default in Tabular Editor. In our sample model we renamed this column Time calc; then we added three items (YTD, QTD, and SPLY for same period last year) by using the New Calculation Item context menu item available by right-clicking on the Time calc column. Each calculation item has a DAX expression, as shown in Figure 9-3. Figure 9-3 Every calculation item has a DAX expression that can be modified in Tabular Editor.

The SELECTEDMEASURE function is the DAX implementation of the <Measure> placeholder we used in the previous DAX pseudo-code. The DAX code for each calculation item is described in the following code. The comment preceding each DAX expression identifies the corresponding calculation item: Note

It is best practice to always expose the business logic through measures in the model. When the model includes calculation groups, the Power BI client does not allow developers to aggregate columns because calculation groups can only be applied to measures, and they do not produce any effect on aggregation functions; they only operate on measures.

```--
-- Calculation Item: YTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESYTD ( 'Date'[Date] )
)

--
-- Calculation Item: QTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESQTD ( 'Date'[Date] )
)

--
-- Calculation Item: SPLY
--
CALCULATE (
SELECTEDMEASURE (),
SAMEPERIODLASTYEAR ( 'Date'[Date] )
)```

With this definition, the user will see a new table named Time Intelligence, with a column named Time calc containing three values: YTD, QTD, and SPLY. The user can create a slicer on that column, or use it on the rows and columns of visuals, as if it were a real column in the model. For example, when the user selects YTD, the engine applies the YTD calculation item to whatever measure is in the report. In Figure 9-4 you can see a matrix containing the Sales Amount measure. Because the slicer selects the YTD variation of the measure, the numbers shown are year-to-date values. Figure 9-4 When the user selects YTD, the values in the matrix represent the YTD variation of the Sales Amount measure.

If on the same report the user selects SPLY, the result will be very different, as you can appreciate in Figure 9-5. Figure 9-5 Selecting SPLY changes the results of the Sales Amount measure because it now uses a different variation. The values are the original Sales Amount values, shifted back one year.

If the user does not select one value or if the user selects multiple values together, then the engine does not apply any variation to the original measure. You can see this in Figure 9-6. Note

The behavior of calculation groups with no selection or with multiple items selected may change in the future. As of April 2019, when multiple calculation items are selected, the behavior is the same as if there were no selection on a calculation group. Nevertheless, this condition might return different results in future versions—for example, raising an error in case of a multiple selection.

Calculation groups can go further than that. At the beginning of this section we introduced four different measures: Sales Amount, Total Cost, Margin, and Sales Quantity. It would be extremely nice if the user could use a slicer in order to select the metric to show and not only the time intelligence calculation to apply. We would like to present a generic report that slices any of the four metrics by month and year, letting the user choose the desired metric. In other words, we want to obtain the report in Figure 9-7. Figure 9-7 The report shows the YTD time intelligence calculation applied to Margin, but the user can choose any other combination through the slicers.

In the example shown in Figure 9-7, the user is browsing the margin amount using a year-to-date variation. Nevertheless, the user can choose any combination of the slicers linked to the two calculation groups, Metric and Time calc.

In order to obtain this report, we created an additional calculation group named Metric, which includes the Sales Amount, Total Cost, Margin, and Sales Quantity calculation items. The expression for each calculation item just evaluates the corresponding measure, as shown in Figure 9-8 for the Sales Amount calculation item. Figure 9-8 The Metric calculation group contains four calculation items; each one simply evaluates a corresponding measure.

When there are multiple calculation groups in the same data model, it is important to define in which order they should be applied by the DAX engine. The Precedence property of the calculation group defines the order of application: the first calculation group applied is the one with the larger value. In order to obtain the desired result, we increased the Precedence property of the Time Intelligence calculation group to 10, as shown in Figure 9-9. As a consequence, the engine applies the Time Intelligence calculation group before the Metric calculation group, which keeps the Precedence property at the default value of zero. We discuss the precedence of calculation groups in more detail later in this chapter. Figure 9-9 The Precedence property defines the order in which each calculation group is applied to a measure.

The following DAX code includes the definition of each calculation item in the Metric calculation group:

```--
-- Calculation Item: Margin
--
[Margin]

--
-- Calculation Item: Sales Amount
--
[Sales Amount]

--
-- Calculation Item: Sales Quantity
--
[Sales Quantity]

--
-- Calculation Item: Total Cost
--
[Total Cost]```

These calculation items are not modifiers of the original measure. Instead, they completely replace the original measure with a new one. To obtain this behavior, we omitted a reference to SELECTEDMEASURE in the expression. SELECTEDMEASURE is used very often in calculation items, but it is not mandatory.

This last example is useful to introduce the first of the many complexities that we will need to address with calculation groups. If the user selects Quantity, then the report shows the quantity, but it still uses the same format strings (with two decimals) as the other measures. Because the Quantity measure is an integer, it would be useful to remove the decimal places or, in general, to adopt a different format string. We discussed earlier the fact that the presence of multiple calculation groups in a calculation requires the definition of a precedence order, as was the case in the previous example. These are the first of several details to consider in order to create useful calculation groups. Note

If you are using Analysis Services, be mindful that adding a calculation group to a model is an operation that requires a refresh of the table corresponding to the calculation group in order to make the calculation items visible to the client. This may prove to be counterintuitive because deploying measures does not require such an update—measures are visible to the clients just after the deployment. However, because calculation groups and items are presented to the client in tables and columns, after the deployment it is necessary to run a refresh operation to populate the internal structures of the tables and columns. In Power BI this operation will likely be handled automatically by the user interface—though this is pure speculation because calculation groups are not present in Power BI at the time of printing.

### Understanding calculation groups

In the previous sections we focused on the use of calculation groups and how to implement them with Tabular Editor. In this section, we describe in more detail the properties and behavior of calculation groups and calculation items.

There are two entities: calculation groups and calculation items. A calculation group is a collection of calculation items, grouped together based on a user-defined criterion. For both calculation groups and calculation items, there are properties that the developer must set correctly. We introduce these entities and their properties here, providing more examples and details in the remaining part of this chapter.

A calculation group is a simple entity, defined by

• The calculation group Name. This is the name of the table that represents the calculation group on the client side.

• The calculation group Precedence. When there are multiple active calculation groups, a number that defines the precedence used to apply each calculation group to a measure reference.

• The calculation group attribute Name. This is the name of the column that includes the calculation items, displayed to the client as unique items available in the column.

A calculation item is a much more sophisticated entity, and here is the list of its properties:

• The calculation item Name. This becomes one value of the calculation group column. Indeed, a calculation item is like one row in the calculation group table.

• The calculation item Expression. A DAX expression that might contain special functions like SELECTEDMEASURE. This is the expression that defines how to apply the calculation item.

• The sort order of the calculation item is defined by the Ordinal value. This property defines how the different calculation items are sorted when presented to the user. It is very similar to the sort-by-column feature of the data model. This feature is not available as of April 2019 but should be implemented before calculation groups are released.

• Format String. If not specified, a calculation item inherits the format string of its base measure. Nevertheless, if the modifier changes the calculation, then it is possible to override the measure format string with the format of the calculation item.

The Format String property is important in order to obtain a consistent behavior of the measures in the model according to the calculation item being applied to them. For example, consider the following calculation group containing two calculation items for time intelligence: year-over-year (YOY) is the difference between a selected period and the same period in the previous year; year-over-year percentage (YOY%) is the percentage of YOY over the amount in the same period in the previous year:

```--
-- Calculation Item: YOY
--
VAR CurrYear =
SELECTEDMEASURE ()
VAR PrevYear =
CALCULATE (
SELECTEDMEASURE (),
SAMEPERIODLASTYEAR ( 'Date'[Date] )
)
VAR Result =
CurrYear - PrevYear
RETURN Result

--
-- Calculation Item: YOY%
--
VAR CurrYear =
SELECTEDMEASURE ()
VAR PrevYear =
CALCULATE (
SELECTEDMEASURE (),
SAMEPERIODLASTYEAR ( 'Date'[Date] )
)
VAR Result =
DIVIDE (
CurrYear - PrevYear,
PrevYear
)
RETURN Result```

The result produced by these two calculation items in a report is correct, but if the Format String property does not override the default format string, then YOY% is displayed as a decimal number instead of as a percentage, as shown in Figure 9-10. Figure 9-10 The two calculation items YOY and YOY% share the same format as the Sales Amount measure.

The example shown in Figure 9-10 displays the YOY evaluation of the Sales Amount measure using the same format string as the original Sales Amount measure. This is the correct behavior to display a difference. However, the YOY% calculation item displays the same amount as a percentage of the value of the previous year. The number shown is correct, but for January one would expect to see −12% instead of −0.12. In this case the expected format string should be a percentage, regardless of the format of the original measure. To obtain the desired behavior, set the Format String property of the YOY% calculation item to percentage, overriding the behavior of the underlying measure. You can see the result in Figure 9-11. If the Format String property is not assigned to a calculation item, the existing format string is used. Figure 9-11 The YOY% calculation item overrides the format of the Sales Amount measure displaying the value as a percentage.

The format string can be defined using a fixed format string or—in more complex scenarios—by using a DAX expression that returns the format string. When one is writing a DAX expression, it becomes possible to refer to the format string of the current measure using the SELECTEDMEASUREFORMATSTRING function, which returns the format string currently defined for the measure. For example, if the model contains a measure that returns the currently selected currency and you want to include the currency symbol as part of the format string, you can use this code to append the currency symbol to the current format string:

`SELECTEDMEASUREFORMATSTRING () & " " & [Selected Currency]`

Customizing the format string of a calculation item is useful to preserve user experience consistency when browsing the model. However, a careful developer should consider that the format string operates on any measure used with the calculation item. When there are multiple calculation groups in a report, the result produced by these properties also depends on the calculation group precedence, as explained in a later section of this chapter.

#### Understanding calculation item application

So far, the description we gave of how a calculation item works has never been extremely precise. The reason is mainly educational: we wanted to introduce the concept of calculation items, without diving too deep into details that might be distracting. Indeed, we said that calculation items can be applied by the user using, for example, a slicer. A calculation item is applied by replacing measure references invoked when there is a calculation item active in the filter context. In this scenario, the calculation item rewrites the measure reference by applying the expression defined in the calculation item itself.

For example, consider the following calculation item:

```--
-- Calculation Item: YTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESYTD ( 'Date'[Date] )
)```

In order to apply the calculation item in an expression, you need to filter the calculation group. You can create this filter using CALCULATE, like in the following example; this is the same technique used by the client tool when using slicers and visuals:

```CALCULATE (
[Sales Amount],
'Time Intelligence'[Time calc] = "YTD"
)```

There is nothing magical about calculation groups: They are tables, and as such they can be filtered by CALCULATE like any other table. When CALCULATE applies a filter to a calculation item, DAX uses the definition of the calculation item to rewrite the expression before evaluating it.

Therefore, based on the definition of the calculation item, the previous code is interpreted as follows:

```CALCULATE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
)
)``` Note

Inside the inner CALCULATE, one can check with ISFILTERED whether the calculation item is filtered or not. In the example, we removed the outer filter on the calculation item for the sake of simplicity, to show that the calculation item has already been applied. Nevertheless, a calculation item retains its filters, and further sub-expressions might still perform the replacement of measures.

Despite being very intuitive in simple examples, this behavior hides some level of complexity. The application of a calculation item replaces a measure reference with the expression of the calculation item. Focus your attention on this last sentence: A measure reference is replaced. Without a measure reference, a calculation item does not apply any modification. For example, the following code is not affected by any calculation item because it does not contain any measure reference:

```CALCULATE (
SUMX ( Sales, Sales[Quantity] * Sales[Net Price] ),
'Time Intelligence'[Time calc] = "YTD"
)```

In this example, the calculation item does not perform any transformation because the code inside CALCULATE does not use any measure. The following code is the one executed after the application of the calculation item:

```CALCULATE (
SUMX ( Sales, Sales[Quantity] * Sales[Net Price] )
)```

If the expression inside CALCULATE contains multiple measure references, all of them are replaced with the calculation item definition. For example, the expression in the following Cost Ratio YTD measure contains two measure references, Total Cost and Sales Amount:

```CR YTD :=
CALCULATE (
DIVIDE (
[Total Cost],
[Sales Amount]
),
'Time Intelligence'[Time calc] = "YTD"
)```

To obtain the actual code executed, replace the measure references with the expansion of the calculation item definition, as in the following CR YTD Actual Code measure:

```CR YTD Actual Code :=
CALCULATE (
DIVIDE (
CALCULATE (
[Total Cost],
DATESYTD ( 'Date'[Date] )
),
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
)
)
)```

In this example, the code generated produces the same result as the next version in the CR YTD Simplified measure, which is more intuitive:

```CR YTD Simplified :=
CALCULATE (
CALCULATE (
DIVIDE (
[Total Cost],
[Sales Amount]
),
DATESYTD ( 'Date'[Date] )
)
)```

These three measures return the same result, as shown in Figure 9-12. Figure 9-12 The CR YTD, CR YTD Actual Code, and CR YTD Simplified measures produce the same result.

Nevertheless, you must be very careful because the CR YTD Simplified measure does not correspond to the actual code generated by the calculation item, which is the code in CR YTD Actual Code. In this very special case, the two versions are equivalent. However, in more complex scenarios the difference is significant, and such a large difference can lead to unintended results that are extremely hard to follow and understand. Let us analyze a couple of examples. In the first example the Sales YTD 2008 2009 measure has two nested CALCULATE functions: the outer CALCULATE sets a filter on the year 2008, whereas the inner CALCULATE sets a filter on the year 2009:

```Sales YTD 2008 2009 :=
CALCULATE (
CALCULATE (
[Sales Amount],
'Date'[Calendar Year] = "CY 2009"
),
'Time Intelligence'[Time calc] = "YTD",
'Date'[Calendar Year] = "CY 2008"
)```

The outer CALCULATE filters the calculation item to the YTD value. Nevertheless, the application of the calculation item does not change the expression because the expression does not directly contain any measure. CALCULATE filters the calculation item, but its application does not lead to any modifications to the code.

Pay attention to the fact that the Sales Amount measure is within the scope of the inner CALCULATE. The application of a calculation item modifies the measures in the current scope of the filter context; it does not affect nested filter context scopes. Those are handled by their own CALCULATE—or equivalent code, such as CALCULATETABLE or context transitions—which may or may not retain the same filter on the calculation item.

When the inner CALCULATE applies its filter context, it does not change the filter status of the calculation item. Therefore, the engine finds that the calculation item is still filtered, and it remains filtered if no other CALCULATE changes it. Same as if it were a regular column. The inner CALCULATE contains a measure reference, and DAX performs the application of the calculation item. The resulting code corresponds to the definition of the Sales YTD 2008 2009 Actual Code measure:

```Sales YTD 2008 2009 Actual Code :=
CALCULATE (
CALCULATE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
),
'Date'[Calendar Year] = "CY 2009"
),
'Date'[Calendar Year] = "CY 2008"
)```

The result of these two measures is visible in Figure 9-13. The selection made by the slicer on the left applies to the matrix in the middle of the figure, which includes the Sales YTD 2008 2009 and Sales YTD 2008 2009 Actual Code measures. However, the selection of the year CY 2008 is overridden by CY 2009. This can be verified by looking at the matrix on the right-hand side, which shows the Sales Amount measure transformed with the YTD calculation item for the CY 2008 and CY 2009 years. The numbers in the center matrix correspond to the CY 2009 column of the matrix on the right. Figure 9-13 The Sales YTD 2008 2009 and Sales YTD 2008 2009 Actual Code measures produce the same result.

The DATESYTD function is applied when the filter context is filtering the year 2009, not 2008. Despite the calculation item being filtered along with the filter for the year 2008, its actual application took place in a different filter context, namely the inner filter context. The behavior is counterintuitive to say the least. The more complex the expression used inside CALCULATE, the harder it becomes to understand how the application works.

The behavior of calculation items leads to one very important best practice: You need to use calculation items to modify an expression if and only if this expression is a single measure. The previous example was only useful to introduce the rule; let us now analyze the best practice with a more complex expression. The next expression computes the number of working days only for the months where there are sales:

```SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
[Sales Amount] > 0, -- Measure reference
[# Working Days]    -- Measure reference
)
)```

This calculation is useful to compute Sales Amount per working day considering only the months with sales. The following example uses this calculation in a more complex expression:

```DIVIDE (
[Sales Amount],  -- Measure reference
SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
[Sales Amount] > 0, -- Measure reference
[# Working Days]    -- Measure reference
)
)
)```

If this expression is executed within an outer CALCULATE that changes the calculation to a YTD, the result is the following new formula that produces an unexpected result:

```Sales WD YTD 2008 :=
CALCULATE (
DIVIDE (
[Sales Amount],  -- Measure reference
SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
[Sales Amount] > 0, -- Measure reference
[# Working Days]    -- Measure reference
)
)
),
'Time Intelligence'[Time calc] = "YTD",
'Date'[Calendar Year] = "CY 2008"
)```

Intuitively, one would expect the previous expression to compute the Sales Amount measure per working days considering all the months before the current one. In other words, one would expect this code to be executed:

```Sales WD YTD 2008 Expected Code :=
CALCULATE (
CALCULATE (
DIVIDE (
[Sales Amount],  -- Measure reference
SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
[Sales Amount] > 0, -- Measure reference
[# Working Days]    -- Measure reference
)
)
) ,
DATESYTD ( 'Date'[Date] )
),
'Date'[Calendar Year] = "CY 2008"
)```

Nevertheless, you might have noticed that we have highlighted the three measure references with a few comments. This was not by chance. The application of a calculation item happens on the measure references, not on the entire expression. Therefore, the code executed by replacing the measure references with the calculation items active in the filter context is very different:

```Sales WD YTD 2008 Actual Code :=
CALCULATE (
DIVIDE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
),
SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
) > 0,
CALCULATE (
[# Working Days],
DATESYTD ( 'Date'[Date] )
)
)
)
),
'Date'[Calendar Year] = "CY 2008"
)```

This latter version of the code produces an abnormal value for the number of working days because it sums the year-to-date of the number of working days for all the months visible in the current context. The chances of producing an inaccurate result are extremely high. When an individual month is selected, the result (by pure luck) is the right one, whereas at the quarter and at the year levels it is hilariously wrong. This is shown in Figure 9-14. Figure 9-14 Different versions of the Sales WD calculation computed for the all the quarters of 2008.

The Sales WD YTD 2008 Expected Code measure returns the correct number for every quarter, whereas the Sales WD YTD 2008 and Sales WD YTD 2008 Actual Code measures return a smaller value. Indeed, the number of working days in the denominator of the ratio is computed as the sum of the year-to-date number of working days for each month in the period.

You can easily avoid this complexity by obeying the best practice: Use CALCULATE with calculation items only to invoke an individual measure. When one authors the Sales WD YTD 2008 Fixed measure that includes the full expression and uses the Sales WD YTD 2008 Fixed measure in a single CALCULATE function, the code is very different and easier to use:

```--
-- Measure Sales WD
--
Sales WD :=
DIVIDE (
[Sales Amount],
SUMX (
VALUES ( 'Date'[Calendar Year month] ),
IF (
[Sales Amount] > 0,
[# Working Days]
)
)
)

--
-- Measure Sales WD YTD 2008 Fixed
-- New version of the Sales WD YTD 2008 measure that applies the YTD calculation item
--
Sales WD YTD 2008 Fixed :=
CALCULATE (
[Sales WD],                             -- Measure reference
'Time Intelligence'[Time calc] = "YTD",
'Date'[Calendar Year] = "CY 2008"
)```

In this case, the code generated by the application of the calculation item is much more intuitive:

```Sales WD YTD 2008 Fixed Actual Code :=
CALCULATE (
CALCULATE (
[Sales WD],
DATESYTD ( 'Date'[Date] )
),
'Date'[Calendar Year] = "CY 2008"
)```

In this latter example the filter provided by DATESYTD surrounds the entire expression, leading to the code that one intuitively expects from the application of the calculation item. The result of the Sales WD YTD 2008 Fixed and Sales WD YTD 2008 Fixed Actual Code measures is visible in Figure 9-14.

For very simple calculations containing simple expressions, it is possible to deviate from this best practice. However, when doing so, the developer must always think twice before creating any measure, because as soon as the complexity of the expression is no longer trivial, the chances of producing wrong calculations become very high.

When using client tools like Power BI, you never have to worry about these details. Indeed, these tools make sure that calculation items get applied the right way because they always invoke single measures as part of the query they execute. Nevertheless, as a DAX developer, you will end up using calculation items as filters in CALCULATE. When you do that, pay attention to the expression used in CALCULATE. If you want to stay on the safe side, use calculation items in CALCULATE to modify a single measure. Never apply calculation items to an expression.

Finally, we suggest you learn calculation items by rewriting the expression manually, applying the calculation item, and writing down the complete code that will be executed. It is a mental exercise that proves very useful in understanding exactly what is happening inside the engine.

#### Understanding calculation group precedence

In the previous section we described how to use CALCULATE to apply a calculation item to a measure. It is possible to apply multiple calculation items to the same measure. Even though each calculation group can only have one active calculation item, the presence of multiple calculation groups can activate multiple calculation items at the same time. This happens when a user uses multiple slicers over different calculation groups, or when a CALCULATE function filters calculation items in different calculation groups. For example, at the beginning of this chapter we defined two calculation groups: one to define the base measure and the other to define the time intelligence calculation to apply to the base measure.

If there are multiple calculation items active in the current filter context, it is important to define which calculation item is applied first, by defining a set of precedence rules. DAX enforces this by making it mandatory to set the Precedence property in a calculation group, in models that have more than one calculation group. This section describes how to correctly set the Precedence property of a calculation group through examples where the definition of the precedence changes the result of the calculations.

To prepare the demonstration, we created two different calculation groups, each one containing only one calculation item:

```-------------------------------------------------------
-- Calculation Group: 'Time Intelligence'[Time calc]
-------------------------------------------------------

--
-- Calculation Item: YTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESYTD ( 'Date'[Date] )
)
-------------------------------------------------------
-- Calculation Group: 'Averages'[Averages]
-------------------------------------------------------

--
-- Calculation Item: Daily AVG
--
DIVIDE (
SELECTEDMEASURE (),
COUNTROWS ( 'Date' )
)```

YTD is a regular year-to-date calculation, whereas Daily AVG computes the daily average by dividing the selected measure by the number of days in the filter context. Both calculation items work just fine, as shown in Figure 9-15, where we use two measures to invoke the two calculation items individually:

```YTD :=
CALCULATE (
[Sales Amount],
'Time Aggregation'[Aggregation] = "YTD"
)

Daily AVG :=
CALCULATE (
[Sales Amount],
'Averages'[Averages] = "Daily AVG"
)``` Figure 9-15 Both Daily AVG and YTD calculation items work just fine when invoked individually in separate measures.

The scenario suddenly becomes more complex when both calculation items are used at the same time. Look at the following Daily YTD AVG measure definition:

```Daily YTD AVG :=
CALCULATE (
[Sales Amount],
'Time Intelligence'[Time calc] = "YTD",
'Averages'[Averages] = "Daily AVG"
)```

The measure invokes both calculation items at the same time, but this raises the issue of precedence. Should the engine apply YTD first and Daily AVG later, or the other way around? In other words, which of these two expressions should be evaluated?

```--
--  YTD is applied first, and then DIVIDE
--
DIVIDE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
),
COUNTROWS ( 'Date' )
)

--
--  DIVIDE is applied first, and then YTD
--
CALCULATE (
DIVIDE (
[Sales Amount],
COUNTROWS ( 'Date' )
),
DATESYTD ( 'Date'[Date] )
)```

It is likely that the second expression is the correct one. Nevertheless, without further information, DAX cannot choose between the two. Therefore, the developer must define the correct order of application of the calculation groups.

The order of application depends on the Precedence property in the two calculation groups: The calculation group with the highest value is applied first; then the other calculation groups are applied according to their Precedence value in a descending order. Figure 9-16 shows the wrong result produced with the following settings:

• Time Intelligence calculation group—Precedence: 0

• Averages calculation group—Precedence: 10

The value of the Daily YTD AVG is clearly wrong in all the months displayed but January. Let us analyze what happened in more depth. Averages has a precedence of 10; therefore, it is applied first. The application of the Daily AVG calculation item leads to this expression corresponding to the Daily YTD AVG measure reference:

```CALCULATE (
DIVIDE (
[Sales Amount],
COUNTROWS ( 'Date' )
),
'Time Intelligence'[Time calc] = "YTD"
)```

At this point, DAX activates the YTD calculation item from the Time Intelligence calculation group. The application of YTD rewrites the only measure reference in the formula, which is Sales Amount. Therefore, the final code corresponding to the Daily YTD AVG measure becomes the following:

```DIVIDE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
),
COUNTROWS ( 'Date' )
)```

Consequently, the number shown is obtained by dividing the Sales Amount measure computed using the YTD calculation item, by the number of days in the displayed month. For example, the value shown in December is obtained by dividing 9,353,814,87 (YTD of Sales Amount) by 31 (the number of days in December). The number should be much lower because the YTD variation should be applied to both the numerator and the denominator of the DIVIDE function used in the Daily AVG calculation item.

To solve the issue, the YTD calculation item must be applied before Daily AVG. This way, the transformation of the filter context for the Date column occurs before the evaluation of COUNTROWS over the Date table. In order to obtain this, we modify the Precedence property of the Time Intelligence calculation group to 20, obtaining the following settings:

• Time Intelligence calculation group—Precedence: 20

• Averages calculation group—Precedence: 10

Using these settings, the Daily YTD AVG measure returns the correct values, as shown in Figure 9-17.

This time, the two application steps are the following: DAX first applies the YTD calculation from the Time Intelligence calculation group, changing the expression to the following:

```CALCULATE (
CALCULATE (
[Sales Amount],
DATESYTD ( 'Date'[Date] )
),
'Averages'[Averages] = "Daily AVG"
)```

Then, DAX applies the Daily AVG calculation item from the Averages calculation group, replacing the measure reference with the DIVIDE function and obtaining the following expression:

```CALCULATE (
DIVIDE (
[Sales Amount],
COUNTROWS ( 'Date' )
),
DATESYTD ( 'Date'[Date] )
)```

The value displayed in December now considers 365 days in the denominator of DIVIDE, thus obtaining the correct number. Before moving further, please consider that, in this example, we followed the best practice of using calculation items with a single measure. Indeed, the first call comes from the visual of Power BI. However, one of the two calculation items rewrote the Sales Amount measure in such a way that the problem arose. In this scenario, following the best practices is not enough. It is mandatory that a developer understand and define the precedence of application of calculation groups very well.

All calculation items in a calculation group share the same precedence. It is impossible to define different precedence values for different calculation items within the same group.

The Precedence property is an integer value assigned to a calculation group. A higher value means a higher precedence of application; the calculation group with the higher precedence is applied first. In other words, DAX applies the calculation groups according to their Precedence value sorted in a descending order. The absolute value assigned to Precedence does not mean anything. What matters is how it compares with the Precedence of other calculation groups. There cannot be two calculation groups in a model with the same Precedence.

Because assigning different Precedence values to multiple calculation groups is mandatory, you must pay attention making this choice when you design a model. Choosing the right Precedence upfront is important because changing the Precedence of a calculation group might affect the existing reports of a model already deployed in production. When you have multiple calculation groups in a model, you should always spend time verifying that the results of the calculations are the results expected with any combination of calculation items. The chances of making mistakes in the definition of the precedence values is quite high without proper testing and validation.

#### Including and excluding measures from calculation items

There are scenarios where a calculation item implements a variation that does not make sense on all the measures. By default, a calculation item applies its effects on all the measures. Nevertheless, the developer might want to restrict which measures are affected by a calculation item.

One can write conditions in DAX that analyze the current measure evaluated in the model by using either ISSELECTEDMEASURE or SELECTEDMEASURENAME. For example, consider the requirement of restricting the measures affected by the Daily AVG calculation item so that a measure computing a percentage is not transformed into a daily average. The ISSELECTEDMEASURE function returns True if the measure evaluated by SELECTEDMEASURE is included in the list of measures specified in the arguments:

```-------------------------------------------------------
-- Calculation Group: 'Averages'[Averages]
-------------------------------------------------------

--
-- Calculation Item: Daily AVG
--
IF (
ISSELECTEDMEASURE (
[Sales Amount],
[Gross Amount],
[Discount Amount],
[Sales Quantity],
[Total Cost],
[Margin]
),
DIVIDE (
SELECTEDMEASURE (),
COUNTROWS ( 'Date' )
)
)```

As you can see, the code specifies the measure on which to compute the daily average, returning blank when the Daily AVG calculation item is applied to any other measure. Now if the requirement is just to exclude specific measures, including any other measure by default, the code can be written this way:

```-------------------------------------------------------
-- Calculation Group: 'Averages'[Averages]
-------------------------------------------------------

--
-- Calculation Item: Daily AVG
--
IF (
NOT ISSELECTEDMEASURE ( [Margin %] ),
DIVIDE (
SELECTEDMEASURE (),
COUNTROWS ( 'Date' )
)
)```

In both cases the Daily AVG calculation item excludes the calculation for the Margin % measure, as shown in Figure 9-18.

Another function that can be used to analyze the selected measure in a calculation item expression is SELECTEDMEASURENAME, which returns a string instead of a Boolean value. This function may be used instead of ISSELECTEDMEASURE, as in the following example:

```-------------------------------------------------------
-- Calculation Group: 'Averages'[Averages]
-------------------------------------------------------

--
-- Calculation Item: Daily AVG
--
IF (
NOT ( SELECTEDMEASURENAME () = "Margin %" ),
DIVIDE (
SELECTEDMEASURE (),
COUNTROWS ( 'Date' )
)
)```

The result would be the same, but the ISSELECTEDMEASURE solution is preferable for several reasons:

• If the measure name is misspelled using a comparison with SELECTEDMEASURENAME, the DAX code simply return False without raising an error.

• If the measure name is misspelled using ISSELECTEDMEASURE, the expression fails with the error Invalid input arguments for ISSELECTEDMEASURE.

• If a measure is renamed in the model, all the expressions using ISSELECTEDMEASURE are automatically renamed in the model editor (formula fixup), whereas the strings compared to SELECTEDMEASURENAME must be updated manually.

The SELECTEDMEASURENAME function should be considered when the business logic of a calculation item must apply a transformation based on an external configuration. For example, the function might be useful when there is a table with a list of measures that should enable a behavior in a calculation item so that the model has an external configuration that can be modified without requiring an update of the DAX code.

### Understanding sideways recursion

DAX calculation items do not provide full recursion. However, there is a limited form of recursion available, which is called sideways recursion. We describe this complex topic through examples. Let us start by understanding what recursion is and why it is important to discuss it. Recursion might occur when a calculation item refers to itself, leading to an infinite loop in the application of calculation items. Let us elaborate on this.

Consider a Time Intelligence calculation group with two calculation items defined as follows:

```-------------------------------------------------------
-- Calculation Group: 'Time Intelligence'[Time calc]
-------------------------------------------------------

--
-- Calculation Item: YTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESYTD ( 'Date'[Date] )
)

--
-- Calculation Item: SPLY
--
CALCULATE (
SELECTEDMEASURE (),
SAMEPERIODLASTYEAR ( 'Date'[Date] )
)```

The requirement is to add a third calculation item that computes the year-to-date in the previous year (PYTD). As you learned in Chapter 8, “Time intelligence calculations,” this can be obtained by mixing two time intelligence functions: DATESYTD and SAMEPERIODLASTYEAR. The following calculation item solves the scenario:

```--
-- Calculation Item: PYTD
--
CALCULATE (
SELECTEDMEASURE (),
DATESYTD ( SAMEPERIODLASTYEAR ( 'Date'[Date] ) )
)```

Given the simplicity of the calculation, this solution is already optimal. Nevertheless, as a mind challenge we can try to author the same code in a different way. Indeed, there already is a YTD calculation item that computes the year-to-date in place; therefore, one could think of using the calculation item instead of mixing time intelligence calculations within the same formula. Look at the following definition of the same PYTD calculation item:

```--
-- Calculation Item: PYTD
--
CALCULATE (
SELECTEDMEASURE (),
SAMEPERIODLASTYEAR ( 'Date'[Date] ),
'Time Intelligence'[Time calc] = "YTD"
)```

The calculation item achieves the same result as the previous definition, but using a different technique. SAMEPERIODLASTYEAR moves the filter context back to the previous year, while the year-to-date calculation is obtained by applying an existing calculation item in the Time calc calculation group: YTD. As previously noted, in this example the code is less readable and needlessly more complex. That said, you can easily imagine that in a more complex scenario the ability to invoke previously defined calculation items might come very handy—to avoid repeating the same code multiple times in your measures.

This is a powerful mechanism to define complex calculations. It comes with some level of complexity that needs to be well understood: recursion. As you have seen in the PYTD calculation item, it is possible to define a calculation item based on another calculation item from the same calculation group. In other words, inside a calculation group certain items can be defined in terms of other items of the same calculation group. If the feature were available without any restriction, this would lead to extremely complex situations where calculation item A depends on B, which depends on C, which in turn can depend on A. The following fictitious example demonstrates the issue:

```-------------------------------------------------------
-- Calculation Group: Infinite[Loop]
-------------------------------------------------------

--
-- Calculation Item: Loop A
--
CALCULATE (
SELECTEDMEASURE (),
Infinite[Loop] = "Loop B"
)

--
-- Calculation Item: Loop B
--
CALCULATE (
SELECTEDMEASURE (),
Infinite[Loop] = "Loop A"
)```

If used in an expression like in the following example, DAX would not be able to apply the calculation items, because A requires the application of B, which in turn requires A, and so on:

```CALCULATE (
[Sales Amount],
Infinite[Loop] = "Loop A"
)```

Some programming languages allow similar circular dependencies to be used in the definition of expressions—typically in functions—leading to recursive definitions. A recursive function definition is a definition where the function is defined in terms of itself. Recursion is extremely powerful, but it is also extremely complex for developers writing code and for the optimizer looking for the best execution path.

For these reasons, DAX does not allow the definition of recursive calculation items. In DAX, a developer can reference another calculation item of the same calculation group, but without referencing the same calculation item twice. In other words, it is possible to use CALCULATE to invoke a calculation item, but the calculation item invoked cannot directly or indirectly invoke the original calculation item. This feature is called sideways recursion. Its goal is not to implement full recursion; Instead, it aims at reusing complex calculation items without providing the full power (and complexity) of recursion. Note

If you are familiar with the MDX language, you should be aware that MDX supports both sideways recursion and full recursion. These capabilities are part of the reasons MDX is more complex a language than DAX. Moreover, full recursion oftentimes leads to bad performance. For these reasons, DAX does not support full recursion by design.

Be mindful that recursion might also occur because a measure sets a filter on a calculation item, not only between calculation items. For example, consider the following definitions of measures (Sales Amount, MA, MB) and calculation items (A and B):

```--
-- Measures definition
--
Sales Amount := SUMX ( Sales, Sales[Quantity] * Sales[Net Price] )
MA := CALCULATE ( [Sales Amount], Infinite[Loop] = "A" )
MB := CALCULATE ( [Sales Amount], Infinite[Loop] = "B" )

-------------------------------------------------------
-- Calculation Group: Infinite[Loop]
-------------------------------------------------------

--
-- Calculation Item: A
--
[MB]

--
-- Calculation Item: B
--
[MA]```

The calculation items do not reference each other. Instead, they reference a measure that, in turn, references the calculation items, generating an infinite loop. We can see this happening by following the calculation item application step by step. Consider the following expression:

```CALCULATE (
[Sales Amount],
Infinite[Loop] = "A"
)```

The application of calculation item A produces the following result:

```CALCULATE (
CALCULATE ( [MB] )
)```

However, the MB measure internally references both Sales Amount and calculation item B; it corresponds to the following code:

```CALCULATE (
CALCULATE (
CALCULATE (
[Sales Amount],
Infinite[Loop] = "B"
)
)
)```

At this point, the application of calculation item B produces the following result:

```CALCULATE (
CALCULATE (
CALCULATE (
CALCULATE ( [MA] )
)
)
)```

Again, the MA measure internally references Sales Amount and calculation item A, and corresponds to the following code:

```CALCULATE (
CALCULATE (
CALCULATE (
CALCULATE (
CALCULATE (
[Sales Amount],
Infinite[Loop] = "A"
)
)
)
)
)```

Now we are back to the initial expression and we potentially enter into an infinite loop of calculation items applied to the expression—although the calculation items do not reference each other. Instead, they reference a measure that, in turn, references the calculation items. The engine is smart enough to detect that, in this case, an infinite loop is present. Therefore, DAX throws an error.

Sideways recursion can lead to very complex expressions that are hard to read and likely to produce unexpected results. Most of the complexity of calculation items with sideways recursion is seen when there are measures that internally apply calculation items with CALCULATE—all the while users change the calculation item through the user interface of the tool, like using a slicer in Power BI.

Our suggestion is to limit the use of sideways recursion in your code as much as you can, though this might mean repeating the same code in multiple places. Only in hidden calculation groups can you safely rely on sideways recursion, so that they can be managed by code but not by users. Keep in mind that Power BI users can define their own measures in a report, and, unaware of a complex topic like recursion, they might generate errors without properly understanding the reason.

### Using the best practices

As we said in the introduction, there are only two best practices to follow to avoid encountering issues with calculation items:

• Use calculation items to modify the behavior of expressions consisting of one measure only. Never use calculation items to change the behavior of more complex expressions.

```--
--   This is a BEST PRACTICE
--
SalesPerWd :=
CALCULATE (
[Sales Amount],                         -- Single measure. This is good
'Time Intelligence'[Time calc] = "YTD"
)

--
--   This is BAD PRACTICE - do not do this!
--
SalesPerWd :=
CALCULATE (
SUMX ( Customer, [Sales Amount] ),      -- Complex expression, it is not a single
'Time Intelligence'[Time calc] = "YTD"  -- measure reference
)```
• Avoid using sideways recursion in any calculation group that remains public and available to users. You can safely use sideways recursion in hidden calculation groups. Still, if you use sideways recursion, pay attention not to introduce full recursion, which would produce an error as a result.

### Conclusions

Calculation groups are an extremely powerful tool to simplify the building of complex models. By letting the developer define variations of measures, calculation groups provide a very compact way of generating hundreds of measures without duplicating code. Moreover, users love calculation groups because they have the option of creating their own combination of calculations.

As a DAX developer, you should understand their power and their limitations. These are the lessons included in this chapter:

• Calculation groups are sets of calculation items.

• Calculation items are variations of a measure. By using the SELECTEDMEASURE function, calculation items have the option of changing the way a calculation goes.

• A calculation item can override the expression and the format string of the current measure.

• If multiple calculation groups are being used in a model, the developer must define the order of application of the calculation items to disambiguate their behavior.

• Calculation items are applied to measure references, not to expressions. Using a calculation item to change the behavior of an expression not consisting of a single measure reference is likely to produce unexpected results. Therefore, it is a best practice to only apply calculation items to expressions made up of a single measure reference.

• A developer can use sideways recursion in the definition of a calculation item, but this suddenly increases the complexity of the whole expression. The developer should limit the use of sideways recursion to hidden calculation groups and avoid sideways recursion in calculation groups that are visible to users.

• Following the best practices is the easiest way to avoid the complexity involved in calculation groups.

Finally, keep in mind that calculation groups are a very recent addition to the DAX language. This is a very powerful feature, and we just started discovering the many uses of calculation groups. We will update the web page mentioned in the introduction of this chapter with references to new articles and blog posts where you can continue to learn about calculation groups.

• No Comment
..................Content has been hidden....................