2.4.6 Pattern Matching Using the LIKE Operator
Advanced Server provides pattern matching using the traditional SQL LIKE operator. The syntax for the LIKE operator is as follows.string LIKE pattern [ ESCAPE escape-character ]string NOT LIKE pattern [ ESCAPE escape-character ]Every pattern defines a set of strings. The LIKE expression returns true if string is contained in the set of strings represented by pattern. As expected, the NOT LIKE expression returns false if LIKE returns true, and vice versa. An equivalent expression is NOT (string LIKE pattern).If pattern does not contain percent signs or underscore, then the pattern only represents the string itself; in that case LIKE acts like the equals operator. An underscore (_) in pattern stands for (matches) any single character; a percent sign (%) matches any string of zero or more characters.LIKE pattern matches always cover the entire string. To match a pattern anywhere within a string, the pattern must therefore start and end with a percent sign.To match a literal underscore or percent sign without matching other characters, the respective character in pattern must be preceded by the escape character. The default escape character is the backslash but a different one may be selected by using the ESCAPE clause. To match the escape character itself, write two escape characters.Note that the backslash already has a special meaning in string literals, so to write a pattern constant that contains a backslash you must write two backslashes in an SQL statement. Thus, writing a pattern that actually matches a literal backslash means writing four backslashes in the statement. You can avoid this by selecting a different escape character with ESCAPE; then a backslash is not special to LIKE anymore. (But it is still special to the string literal parser, so you still need two of them.)It’s also possible to select no escape character by writing ESCAPE ''. This effectively disables the escape mechanism, which makes it impossible to turn off the special meaning of underscore and percent signs in the pattern.