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%\';
+--------------------+------         


        

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%';

Answer 2

I had a similar issue with space and hyphens while matching strings with exact match:

SELECT id FROM location WHERE name = 'IND - HQ';

The above query didn't return any records in MySQL. I had to escape the spaces and hyphens and use LIKE instead of exact match with equals (=) as follows:

SELECT id FROM location WHERE name LIKE 'IND_\-_HQ';