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SELECT CITY, SUM(SALES) FROM OFFICES, SALESREPS WHERE OFFICE =
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Subqueries and Query Expressions
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CASE WHEN (REP_OFFICE IS NOT NULL) THEN REP_OFFICE ELSE (SELECT REP_OFFICE FROM SALESREPS AS MGRS WHERE MGRS.EMPL_NUM = MANAGER)
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The SQL2 standard provides a shorthand version of the CASE expression for the common situation where you want to compare a test value of some kind to a sequence of data values (usually literals). This version of the CASE syntax is shown in Figure 9-11. Instead of repeating a search condition of the form: test_value = value1 in each WHEN clause, it lets you specify the test_value calculation once. For example, suppose you wanted to generate a list of all of the offices, showing the names of their managers and the cities and states where they are located. The sample database doesn t include state names, so the query must generate this information itself. Here is a query, with a CASE expression in the SELECT list, that does the job:
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SELECT NAME, CITY, CASE OFFICE WHEN WHEN WHEN WHEN WHEN FROM OFFICES, SALESREPS WHERE MGR = EMPL_NUM 11 12 13 21 22 THEN THEN THEN THEN THEN 'New York' 'Illinois' 'Georgia' 'California' 'Colorado'
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The COALESCE Expression (SQL2)
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One of the most common uses for the decision-making capability of the CASE expression is for handling NULL values within the database. For example, it s frequently desirable to have a NULL value from the database represented by some literal value (such as the word missing ) or by some default value when using SQL to generate a report. Here is a report that lists the salespeople and their quotas. If a salesperson has not yet been assigned a quota, assume that the salesperson s actual year-to-date sales should be listed
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Figure 9-11.
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SQL2 CASE expression alternative syntax
SQL: The Complete Reference
instead. If for some reason the actual year-to-date sales are also NULL (unknown), then a zero amount should be listed. The CASE statement generates the desired IF THEN ELSE logic:
SELECT NAME, CASE WHEN (QUOTA IS NOT NULL) THEN QUOTA WHEN (SALES IS NOT NULL) THEN SALES ELSE 0.00 FROM SALESREPS
This type of NULL-handling logic is needed frequently, so the SQL2 standard includes a specialized form of the CASE expression, the COALESCE expression, to handle it. The syntax for the COALESCE expression is shown in Figure 9-12. The processing rules for the COALESCE expression are very straightforward. The DBMS examines the first value in the list. If its value is not NULL, it becomes the value of the COALESCE expression. If the first value is NULL, the DBMS moves to the second value and checks to see whether it is NULL. If not, it becomes the value of the expression. Otherwise, the DBMS moves to the third value, and so on. Here is the same example just given, expressed with the COALESCE expression instead of a CASE expression:
SELECT NAME, COALESCE (QUOTA, SALES, 0.00) FROM SALESREPS
As you can see by comparing the two queries, the simplicity of the COALESCE syntax makes it easier to see, at a glance, the meaning of the query. However, the operation of the two queries is identical. The COALESCE expression adds simplicity, but no new capability, to the SQL2 language.
The NULLIF Expression (SQL2)
Just as the COALESCE expression is used to eliminate NULL values when they are not desired for processing, sometimes you may need to create NULL values. In many data processing applications (especially older ones that were developed before relational databases were popular), missing data is not represented by NULL values. Instead, some special code value that is otherwise invalid is used to indicate that the data is missing. For example, suppose that in the sample database, the situation where a salesperson had not yet been assigned a manager was indicated by a zero (0) value in the MANAGER
Figure 9-12.
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