Why does "_" (underscore) match "-" (hyphen)?

Question

I have to look for a PDF manual using this query:

root@localhost:test> select * from a where name like '%taz_manual%.pdf%'; +--------------------+------------------+-------------+ | name               | description      |        size |  +--------------------+------------------+-------------+ | taz-manual-1.1.pdf | Manual v1.0 TA-Z |    31351902 | | taz-manual-0.2.pdf | Manual v1.0 T1-A |     3578278 | | taz_manual-2.0.pdf | Manual v2.0 GA-X |   542578278 | etc........ +--------------------+------------------+-------------+ 132 row in set (0.00 sec) 

Why am I seeing the the one with dashes when I specify the name to be taz_manual%.pdf?

Answer 1

Because the underscore _ is a wildcard like the percent %, except that it only looks for one character.

SQL pattern matching enables you to use "_" to match any single character and "%" to match an arbitrary number of characters (including zero characters).

(From section 3.3.4.7. Pattern Matching in the MySQL documentation.)

If you want to use the underscore in like as a literal, you have to escape it:

select * from a where name like '%taz\_manual%.pdf%';