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PART II
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For each row of query results, the DBMS evaluates the CASE expression by first comparing the credit limit with $60,000, and if the comparison is TRUE, returning an A in the second column of query results. If that comparison fails, the comparison to $30,000 is made and a B is returned if this second comparison is TRUE. Otherwise, the third column of query results returns a C. This is a very simple example of a CASE expression. The results of the CASE expression are all literals here, but in general, they can be any SQL expression. Similarly, there is no requirement that the tests in each WHEN clause be similar, as they are here. The CASE expression can also appear in other clauses of a query provided the SQL implementation supports such use. Here is an example of a query where the CASE expression is useful in the WHERE clause. Suppose you want to find the total of the salespeople s sales, by office. If a salesperson is not yet assigned to an office, that person
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Part II:
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should be included in the total for his or her manager s office. Here is a query that generates the appropriate office groupings:
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SELECT CITY, SUM(SALESREPS.SALES) FROM OFFICES, SALESREPS WHERE OFFICE = CASE WHEN (REP_OFFICE IS NOT NULL) THEN REP_OFFICE ELSE (SELECT REP_OFFICE FROM SALESREPS MGRS WHERE MGRS.EMPL_NUM = MANAGER) END GROUP BY CITY;
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The SQL standard provides a shorthand version of the CASE expression for the common situation where you want to compare a test value of some kind with 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:
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test_value = value1
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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. 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 END AS STATE 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
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 instead. If for
CASE
test-value
WHEN value THEN
result-expression NULL ELSE result-expression NULL
FIGURE 9-11
CASE expression alternate syntax
9:
Subqueries and Query Expressions
FIGURE 9-12 COALESCE expression syntax diagram
COALESCE (
value-expression ,
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 END AS ADJUSTED_QUOTA FROM SALESREPS;
PART II
This type of NULL-handling logic is needed frequently, so the SQL 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 SQL. 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 column instead of a NULL value. In some circumstances, you will want to detect this situation within a SQL query and substitute the NULL value for the zero code. The NULLIF expression, shown in Figure 9-13, is used for this purpose. When the DBMS encounters a NULLIF expression, it examines the first value (usually a column name) and compares it with the
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