Why Does MySQL Require Double Backslashes to Escape a Backslash in LIKE Queries But Not in Equals (=) Queries?

Understanding MySQL's Slash () Escaping in Queries

It's a common practice in MySQL to escape special characters, including the backslash (), in queries to prevent interpretation conflicts. However, a curious anomaly arises when comparing the behavior of the equals operator (=) and the LIKE operator with regards to the backslash.

The Equal Operator (=)

In the case of the equals operator, no additional backslash is required to escape the backslash within the search term. For instance, the following query retrieves the row where the title column contains the value "test".

SELECT * FROM `titles` WHERE title = 'test\';

The MySQL parser automatically interprets the backslash in the search term as a literal character, hence the successful retrieval.

The LIKE Operator

Unlike the equals operator, the LIKE operator requires an additional backslash to escape the backslash in the search term. This is because the LIKE operator performs a pattern match, and the backslash acts as an escape character by default in SQL. Thus, the following query retrieves the same row as the previous one:

SELECT * FROM `titles` WHERE title LIKE 'test\\';

In this query, the double backslashes ensure that the single backslash in the search term is interpreted literally as a backslash character.

Explanation

The reason behind this behavior is rooted in how MySQL processes LIKE patterns. The parser initially strips the backslashes from the search term. However, when the pattern match is subsequently performed, the backslashes are removed once again. This leaves a single backslash to be matched against the pattern. To ensure that the backslash is treated as a literal character, the additional backslash is required.

Changing the Escape Character

MySQL allows you to specify a different escape character for LIKE patterns using the ESCAPE clause. For example, the following query uses the pipe (|) character as the escape character:

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

In this example, the single backslash in the search term is interpreted literally because it is preceded by the specified escape character.

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