Sorting Retrieved Data
This chapter is from the book
This chapter is from the book
This chapter is from the book
In this lesson, you will learn how to use the SELECT statement's ORDER BY clause to sort retrieved data as needed.
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.
INPUT
SELECT prod_name FROM Products;
INPUT
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 King doll Queen doll
Actually, the retrieved data is not displayed in a mere random order. If unsorted, data will typically 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.
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:
INPUT
SELECT prod_name FROM Products ORDER BY prod_name;
ANALYSIS
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 alphabetically by the prod_name column. The results are as follows:
OUTPUT
prod_name -------------------- 12 inch teddy bear 18 inch teddy bear 8 inch teddy bear Bird bean bag toy Fish bean bag toy King doll Queen doll 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. Using clauses out of order will generate an error message.
TIP
Sorting by Nonselected Columns More often than not, the columns used in an ORDER BY clause will be ones that were selected for display. However, this is actually not required, and it is perfectly legal to sort data by a column that is not retrieved.