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The LIKE Operator
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You typically use the LIKE operator in the WHERE clause of your queries in combination with a search pattern. In the example shown in Listing 4-27, the query returns all courses that have something to do with SQL, using the search pattern %SQL%. Listing 4-27. Using the LIKE Operator with the Percent Character select * from courses where description LIKE '%SQL%'; CODE ---SQL PLS DESCRIPTION -----------------------------Introduction to SQL Introduction to PL/SQL TYP DURATION --- -------GEN 4 BLD 1
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Two characters have special meaning when you use them in a string (the search pattern) after the LIKE operator. These two characters are commonly referred to as wildcards: %: A percent sign after the LIKE operator means zero, one, or more arbitrary characters (see Listing 4-27). _: An underscore after the LIKE operator means exactly one arbitrary character.
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Note If the LIKE operator (with its two wildcard characters) provides insufficient search possibilities, you can use the REGEXP_LIKE function and regular expressions. See 5 for information about using regular expressions.
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The query shown in Listing 4-28 returns all employees with an uppercase A as the second character in their name.
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Listing 4-28. Using the LIKE Operator with the Percent and Underscore Characters select empno, init, ename from employees where ename like '_A%'; EMPNO -------7521 7654 INIT ----TF P ENAME -------WARD MARTIN
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Just like the BETWEEN and IN operators, the LIKE operator also features a built-in negation option; in other words, you can use WHERE NOT LIKE . The following queries show two special cases: one using LIKE without wildcards and one using the % character without the LIKE operator. select * from employees where ename like 'BLAKE' select * from employees where ename = 'BL%' Both queries will be executed by Oracle, without any complaints or error messages. However, in the first example, we could have used the equal sign (=) instead of the LIKE operator to get the same results. In the second example, the percent sign (%) has no special meaning, since it doesn t follow the LIKE operator, so it is very likely we would get back the no rows selected message. If you really want to search for actual percent sign or underscore characters with the LIKE operator, you need to suppress the special meaning of those characters. You can do this with the ESCAPE option of the LIKE operator, as demonstrated in Listing 4-29. Listing 4-29. Using the ESCAPE Option of the LIKE Operator select empno, begindate, comments from history where comments like '%0\%%' escape '\'; EMPNO -------7566 7788 BEGINDATE ----------01-JUN-1989 15-APR-1985 COMMENTS ---------------------------------------------------From accounting to human resources; 0% salary change Transfer to human resources; 0% salary raise
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The WHERE clause in Listing 4-29 searches for 0% in the COMMENTS column of the HISTORY table. The backslash (\) suppresses the special meaning of the second percent sign in the search string. Note that you can pick a character other than the backslash to use as the ESCAPE character.
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4.7 CASE Expressions
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You can tackle complicated procedural problems with CASE expressions. Oracle supports two CASE expression types: simple CASE expressions and searched CASE expressions. Figure 4-3 illustrates the syntax of the simple CASE expression. With this type of CASE expression, you specify an input expression to be compared with the values in the WHEN ... THEN loop. The implicit
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comparison operator is always the equal sign. The left operand is always the input expression, and the right operand is the value from the WHEN clause.
Figure 4-3. Simple CASE expression syntax diagram Figure 4-4 shows the syntax of the searched CASE expression. The power of this type of CASE expression is that you don t specify an input expression, but instead specify complete conditions in the WHEN clause. Therefore, you have the freedom to use any logical operator in each individual WHEN clause.
Figure 4-4. Searched CASE expressions syntax diagram CASE expressions are evaluated as follows: Oracle evaluates the WHEN expressions in the order in which you specified them, and returns the THEN result of the first condition evaluating to TRUE. Note that Oracle does not evaluate the remaining WHEN clauses; therefore, the order of the WHEN expressions is important. If none of the WHEN expressions evaluates to TRUE, Oracle returns the ELSE expression. If you didn t specify an ELSE expression, Oracle returns a null value.
Obviously, you must handle datatypes in a consistent way. The input expressions and the THEN results in the simple CASE expression (Figure 4-3) must have the same datatype, and in both CASE expression types (Figures 4-3 and 4-4), the THEN results should have the same datatype, too. Listing 4-30 shows a straightforward example of a simple CASE expression, which doesn t require any explanation.
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