Why Does MySQL Require Double Escaping for Backslashes in LIKE Queries but Not in Equality Comparisons?

MySQL Escape Character and LIKE Operator: Why Double Escaping for ''

Consider the following MySQL query:

(SELECT * FROM `titles` WHERE title = 'test\')
UNION ALL
(SELECT * FROM `titles` WHERE title LIKE 'test\\')

The result shows that both queries successfully match the row with 'test' title, even though an extra backslash is added in the LIKE clause ('test\'). This behavior raises the question: why is an additional backslash () required for the LIKE operator but not for the = (equals) operator?

Escaping '' in String Literals

MySQL utilizes a character ('') as an escape character in strings. It signifies that the following character has a special meaning. In the literal 'test', the backslash escapes the backslash character itself, resulting in 'test' as the actual string. This behavior is consistent with most programming languages and SQL implementations.

LIKE Operator and Escape Characters

The LIKE operator is designed for pattern matching in strings. However, MySQL specifically designates a backslash () as a wildcard character, which matches any single character. This behavior introduces a potential conflict because a literal backslash () must be escaped to be treated as a character.

To resolve this conflict, MySQL applies double escaping to the backslash character when used in LIKE patterns. This means that every backslash in the pattern must be doubled to be treated as a literal backslash. For example, the pattern 'test' matches strings with 'test' while 'test\' matches strings with 'test'.

Modifying the Escape Character

The default escape character for the LIKE operator can be modified using the ESCAPE clause. For instance, the following query uses '|' as the escape character, allowing a single backslash to be treated as literal:

SELECT * FROM `titles` WHERE title LIKE 'test\' ESCAPE '|'

Conclusion

MySQL's double escaping mechanism for the backslash character in LIKE patterns is necessary to avoid conflicts with the wildcard behavior of the backslash. While regular string escaping applies a single level of escape, the LIKE operator requires double escaping to differentiate between literal backslashes and wildcard patterns. Understanding this behavior ensures accurate pattern matching when using the LIKE operator in MySQL.

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