ProblemYou want to perform a pattern match rather than a literal comparison. SolutionUse the LIKE operator and an SQL pattern, described in this section. Or use a regular-expression pattern match, described in Section 5.11. DiscussionPatterns are strings that contain special characters. These are known as metacharacters because they stand for something other than themselves. MySQL provides two kinds of pattern matching. One is based on SQL patterns and the other on regular expressions. SQL patterns are more standard among different database systems, but regular expressions are more powerful. The two kinds of pattern match uses different operators and different sets of metacharacters. This section describes SQL patterns. Section 5.11 describes regular expressions. The example here uses a table named metal that contains the following rows: +----------+ | name | +----------+ | copper | | gold | | iron | | lead | | mercury | | platinum | | silver | | tin | +----------+ SQL pattern matching uses the LIKE and NOT LIKE operators rather than = and != to perform matching against a pattern string. Patterns may contain two special metacharacters: _ matches any single character, and % matches any sequence of characters, including the empty string. You can use these characters to create patterns that match a variety of values:
An SQL pattern matches successfully only if it matches the entire comparison value. Of the following two pattern matches, only the second succeeds: 'abc' LIKE 'b' 'abc' LIKE '%b%' To reverse the sense of a pattern match, use NOT LIKE. The following statement finds strings that contain no i characters: mysql> SELECT name FROM metal WHERE name NOT LIKE '%i%'; +---------+ | name | +---------+ | copper | | gold | | lead | | mercury | +---------+ SQL patterns do not match NULL values. This is true both for LIKE and for NOT LIKE: mysql> SELECT NULL LIKE '%', NULL NOT LIKE '%'; +---------------+-------------------+ | NULL LIKE '%' | NULL NOT LIKE '%' | +---------------+-------------------+ | NULL | NULL | +---------------+-------------------+
In some cases, pattern matches are equivalent to substring comparisons. For example, using patterns to find strings at one end or the other of a string is like using LEFT( ) or RIGHT( ):
If you're matching against a column that is indexed and you have a choice of using a pattern or an equivalent LEFT( ) expression, you'll likely find that the pattern match is faster. MySQL can use the index to narrow the search for a pattern that begins with a literal string. With LEFT( ), it cannot. |