Sorting Retrieved Data from SQL Databases
Date: Jun 18, 2004
Sample Chapter is provided courtesy of Sams.
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.
Sorting by Multiple Columns
It is often necessary to sort data by more than one column. For example, if you are displaying an employee list, you might want to display it sorted by last name and first name (first by last name, and then within each last name sort by first name). This would be useful if there are multiple employees with the same last name.
To sort by multiple columns, simply specify the column names separated by commas (just as you do when you are selecting multiple columns).
The following code retrieves three columns and sorts the results by two of them—first by price and then by name.
INPUT
SELECT prod_id, prod_price, prod_name FROM Products ORDER BY prod_price, prod_name;
OUTPUT
prod_id prod_price prod_name ------- ---------- -------------------- BNBG02 3.4900 Bird bean bag toy BNBG01 3.4900 Fish bean bag toy BNBG03 3.4900 Rabbit bean bag toy RGAN01 4.9900 Raggedy Ann BR01 5.9900 8 inch teddy bear BR02 8.9900 12 inch teddy bear RYL01 9.4900 King doll RYL02 9.4900 Queen doll BR03 11.9900 18 inch teddy bear
It is important to understand that when you are sorting by multiple columns, the sort sequence is exactly as specified. In other words, using the output in the example above, the products are sorted by the prod_name column only when multiple rows have the same prod_price value. If all the values in the prod_price column had been unique, no data would have been sorted by prod_name.
Sorting by Column Position
In addition to being able to specify sort order using column names, ORDER BY also supports ordering specified by relative column position.
The best way to understand this is to look at an example:
INPUT
SELECT prod_id, prod_price, prod_name FROM Products ORDER BY 2, 3;
OUTPUT
prod_id prod_price prod_name ------- ---------- -------------------- BNBG02 3.4900 Bird bean bag toy BNBG01 3.4900 Fish bean bag toy BNBG03 3.4900 Rabbit bean bag toy RGAN01 4.9900 Raggedy Ann BR01 5.9900 8 inch teddy bear BR02 8.9900 12 inch teddy bear RYL01 9.4900 King doll RYL02 9.4900 Queen doll BR03 11.9900 18 inch teddy bear
ANALYSIS
As you can see, the output is identical to that of the query above. The difference here is in the ORDER BY clause. Instead of specifying column names, the relative positions of selected columns in the SELECT list are specified. ORDER BY 2 means sort by the second column in the SELECT list, the prod_price column. ORDER BY 2, 3 means sort by prod_price and then by prod_name.
The primary advantage of this technique is that it saves retyping the column names. But there are some downsides too. First, not explicitly listing column names increases the likelihood of you mistakenly specifying the wrong column. Second, it is all too easy to mistakenly reorder data when making changes to the SELECT list (forgetting to make the corresponding changes to the ORDER BY clause). And finally, obviously you cannot use this technique when sorting by columns that are not in the SELECT list.
TIP
Sorting by Nonselected Columns Obviously, this technique cannot be used when sorting by columns that do not appear in the SELECT list. However, you can mix and match actual column names and relative column positions in a single statement if needed.
Specifying Sort Direction
Data sorting is not limited to ascending sort orders (from A to Z). Although this is the default sort order, the ORDER BY clause can also be used to sort in descending order (from Z to A). To sort by descending order, the keyword DESC must be specified.
The following example sorts the products by price in descending order (most expensive first):
INPUT
SELECT prod_id, prod_price, prod_name FROM Products ORDER BY prod_price DESC;
OUTPUT
prod_id prod_price prod_name ------- ---------- -------------------- BR03 11.9900 18 inch teddy bear RYL01 9.4900 King doll RYL02 9.4900 Queen doll BR02 8.9900 12 inch teddy bear BR01 5.9900 8 inch teddy bear RGAN01 4.9900 Raggedy Ann BNBG01 3.4900 Fish bean bag toy BNBG02 3.4900 Bird bean bag toy BNBG03 3.4900 Rabbit bean bag toy
But what if you were to sort by multiple columns? The following example sorts the products in descending order (most expensive first), plus product name:
INPUT
SELECT prod_id, prod_price, prod_name FROM Products ORDER BY prod_price DESC, prod_name;
OUTPUT
prod_id prod_price prod_name ------- ---------- -------------------- BR03 11.9900 18 inch teddy bear RYL01 9.4900 King doll RYL02 9.4900 Queen doll BR02 8.9900 12 inch teddy bear BR01 5.9900 8 inch teddy bear RGAN01 4.9900 Raggedy Ann BNBG02 3.4900 Bird bean bag toy BNBG01 3.4900 Fish bean bag toy BNBG03 3.4900 Rabbit bean bag toy
ANALYSIS
The DESC keyword only applies to the column name that directly precedes it. In the example above, DESC was specified for the prod_price column, but not for the prod_name column. Therefore, the prod_price column is sorted in descending order, but the prod_name column (within each price) is still sorted in standard ascending order.
TIP
Sorting Descending on Multiple Columns If you want to sort descending on multiple columns, be sure each column has its own DESC keyword.
It is worth noting that DESC is short for DESCENDING, and both keywords may be used. The opposite of DESC is ASC (or ASCENDING), which may be specified to sort in ascending order. In practice, however, ASC is not usually used because ascending order is the default sequence (and is assumed if neither ASC nor DESC are specified).
TIP
Case Sensitivity and Sort Orders When you are sorting textual data, is A the same as a? And does a come before B or after Z? These are not theoretical questions, and the answers depend on how the database is set up.
In dictionary sort order, A is treated the same as a, and that is the default behavior for most Database Management Systems. However, most good DBMSs enable database administrators to change this behavior if needed. (If your database contains lots of foreign language characters, this might become necessary.)
The key here is that if you do need an alternate sort order, you cannot accomplish it with a simple ORDER BY clause. You must contact your database administrator.
Summary
In this lesson, you learned how to sort retrieved data using the SELECT statement's ORDER BY clause. This clause, which must be the last in the SELECT statement, can be used to sort data on one or more columns as needed.