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Working with Functions and Value Expressions
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You do not have to put the various components of the CASE value expression on separate lines, as I have done here. I do it this way to clearly show you each component. It also makes the code more readable to anyone reviewing it.
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In this SELECT statement, the different values in the CD_CATEGORY column are renamed to more useful names. Notice that you do not need to repeat the column names to the right of the THEN keyword. The predicate construction is assumed by the context of the clause. When you execute this statement, you receive the following query results:
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CD_NAME ----------------------------Famous Blue Raincoat Blue Court and Spark Past Light That Christmas Feeling Patsy Cline: 12 Greatest Hits Out of Africa Leonard Cohen The Best Of Fundamental Blues on the Bayou CD_CATEGORY -----------Folk Rock Classic Pop Classic Pop New Age Christmas Country Soundtrack Folk Rock Blues Blues
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As you can see, only user-friendly names appear in the CD_CATEGORY column. If any of the original values had not met the condition defined in the WHEN/THEN clauses, a null value would be inserted in the query results.
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Can you use a CASE value expression anywhere other than a SELECT statement Another handy use for the CASE value expression is in the SET clause of an UPDATE statement. For example, suppose you want to update the values in the ON_ORDER column in the CD_TRACKING table (shown in Figure 10-3). You can update those values by specifying specific conditions in a CASE expression:
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UPDATE CD_TRACKING SET ON_ORDER = CASE WHEN ON_ORDER < 6 THEN ON_ORDER + 4 WHEN ON_ORDER BETWEEN 6 AND 8 THEN ON_ORDER + 2 ELSE ON_ORDER END
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(continued)
SQL: A Beginner s Guide
This statement will add 4 to the ON_ORDER values that are less than 6, and it will add 2 to the ON_ORDER values that fall within the range of 6 through 8. Otherwise, no additional rows will be changed.
Q: A:
Can you reference more than one column in a CASE value expression Yes, you can reference more than one column. Suppose you want to update ON_ORDER values, but base those updates on CD_CATEGORY values. You can create a statement similar to the following:
UPDATE CD_TRACKING SET ON_ORDER = CASE WHEN CD_CATEGORY = 'CPOP' THEN ON_ORDER * 3 WHEN CD_CATEGORY = 'BLUS' THEN ON_ORDER * 2 ELSE ON_ORDER END
In this statement, ON_ORDER values are multiplied by 3 when CD_CATEGORY values equal CPOP, and ON_ORDER values are multiplied by 2 when CD_CATEGORY values equal BLUS. Otherwise, no values are changed.
Q: A:
Are there any performance implications when using a CASE value expression While there are no inherent performance issues regarding the use of CASE value expressions, it is possible to get carried away. In general, the more complex the statement, especially in terms of nesting and elaborate conditional logic, the more resources that will be required to parse and execute the statement.
Using the CAST Value Expression
The CAST value expression serves a much different purpose than the CASE expression. The CAST expression allows you to change a value s data type for a literal value or when retrieving that value from your database. However, it does not change the data type of the source column. This is particularly useful when working with programming languages in which data types do not match up and you need to use a common denominator to work with the value. To use the CAST value expression, you must specify the CAST keyword, and, in parentheses, provide the column name, the AS keyword, and the new data type, in that order. To illustrate this, let s return to the SALES_DATES table shown in Figure 10-2. The table includes the COMPACT_DISC column and the DATE_SOLD column. The DATE_SOLD
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