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

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?

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%';

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';