Sorting Data
As you learned in the last lesson, the following SQL statement returns a single column from a database table. But look at the output. The data appears to be displayed in no particular order at all.
SELECT prod_name FROM Products;
prod_name -------------------- Fish bean bag toy Bird bean bag toy Rabbit bean bag toy 8 inch teddy bear 12 inch teddy bear 18 inch teddy bear Raggedy Ann
Actually, the retrieved data is not displayed in a mere random order. If left unsorted, data will be displayed in the order in which it appears in the underlying tables. This could be the order in which the data was added to the tables initially. However, if data was subsequently updated or deleted, the order will be affected by how the DBMS reuses reclaimed storage space. The end result is that you cannot (and should not) rely on the sort order if you do not explicitly control it. Relational database design theory states that the sequence of retrieved data cannot be assumed to have significance if ordering was not explicitly specified.
Note
Clause SQL statements are made up of clauses, some required and some optional. A clause usually consists of a keyword and supplied data. An example of this is the SELECT statement's FROM clause, which you saw in the last lesson.
To explicitly sort data retrieved using a SELECT statement, the ORDER BY clause is used. ORDER BY takes the name of one or more columns by which to sort the output. Look at the following example:
SELECT prod_name FROM Products ORDER BY prod_name;
This statement is identical to the earlier statement, except it also specifies an ORDER BY clause instructing the Database Management System software to sort the data by the prod_name column. The results are as follows:
prod_name -------------------- 12 inch teddy bear 18 inch teddy bear 8 inch teddy bear Bird bean bag toy Fish bean bag toy Rabbit bean bag toy Raggedy Ann
Caution
Position of ORDER BY Clause When specifying an ORDER BY clause, be sure that it is the last clause in your SELECT statement. If it is not the last clause, an error will be generated.
Tip
Sorting by Non-Selected Columns Although more often than not the columns used in an ORDER BY clause will be ones selected for display, this is actually not required. It is perfectly legal to sort data by a column that is not retrieved.