Match-Merging: Details

Match-Merging Syntax

Match-merging combines observations from two or more data sets into a single observation in a new data set according to the values of a common variable.
When match-merging, use the MERGE statement rather than the SET statement to combine data sets.
Syntax, DATA step for match-merging:
DATA output-SAS-data-set;
MERGE SAS-data-set-1 SAS-data-set-2;
BY <DESCENDING> variable(s);
RUN;
  • output-SAS-data-set names the data set to be created.
  • SAS-data-set-1 and SAS-data-set-2 specify the data sets to be read.
  • variable(s) in the BY statement specifies one or more variables whose values are used to match observations.
  • DESCENDING indicates that the input data sets are sorted in descending order (largest to smallest numerically, or reverse alphabetical for character variables) by the variable that is specified. If you have more that one variable in the BY statement, DESCENDING applies only to the variable that immediately follows it. The defaults sort order is ASCENDING.
Tip
Each input data set in the MERGE statement must be sorted in order of the values of the BY variable or variables, or it must have an appropriate index. Each BY variable must have the same type in all data sets to be merged.
Tip
You cannot use the DESCENDING option with indexed data sets because indexes are always stored in ascending order.

How Match-Merging Selects Data

During match-merging SAS sequentially checks each observation of each data set to see whether the BY values match and then writes the combined observation to the new data set. 
data merged; 
   merge a b; 
   by num; 
run;
Figure 13.11 How Match-Merging Selects Data
How Match-Merging Selects Data
Basic DATA step match-merging produces an output data set that contains values from all observations in all input data sets. You can add statements and options to select only matching observations.
If an input data set does not have any observations for a particular value of the BY-variable, then the observation in the output data set contains missing values for the variables that are unique to that input data set.
Tip
In match-merging, often one data set contains unique values for the BY-variable and other data sets contain multiple values for the BY-variable.

Example: Using Match-Merging to Combine Data Sets

The data sets Clinic.Demog and Clinic.Visit have been sorted as follows: 
proc sort data=clinic.demog; 
   by id; 
run; 
proc print data=clinic.demog; 
run;
Figure 13.12 HTML Output: Sorting Clinic.Demog
Example: Sorting clinic.demog
proc sort data=clinic.visit; 
   by id; 
run; 
proc print data=clinic.visit; 
run;
Figure 13.13 HTML Output: Sorting Clinic.Visit
Example: Sorting clinic.visit
You can then submit this DATA step to create Clinic.Merged by merging Clinic.Demog and Clinic.Visit according to values of the variable ID. 
data clinic.merged; 
   merge clinic.demog clinic.visit; 
   by id; 
run; 
proc print data=clinic.merged; 
run;
Note: All observations, including unmatched observations and observations that have missing data, are written to the output data set.
Figure 13.14 HTML Output: Match-Merging Output
All observations including observations with missing data are written to the output data set shown in the output graphic.

Example: Merge in Descending Order

The example above illustrates merging two data sets that are sorted in ascending order of the BY variable ID. To sort the data sets in descending order and then merge them, you can submit the following program. 
proc sort data=clinic.demog; 
   by descending id; 
run; 
proc sort data=clinic.visit; 
   by descending id; 
run; 
data clinic.merged; 
   merge clinic.demog clinic.visit; 
   by descending id; 
run; 
proc print data=clinic.merged; 
run;
Note: Specify the DESCENDING option in the BY statements in both the PROC SORT steps and the DATA step. If you omit the DESCENDING option in the DATA step, you generate error messages about improperly sorted BY variables.
Figure 13.15 HTML Output: Merge in Descending Order
The data sets are merged in descending order of the BY variable ID.
Last updated: January 10, 2018
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