Multiple and single indexes

Question

I\'m kinda ashamed of asking this since I\'ve been working with MySQL for years, but oh well.

I have a table with two fields, a and b. I will b

Answer 1

SQL will choose the index that best covers the query. An index on A, B will cover the query for both case 1 and 3, but not for 2 (since the primary index column is A)

So to cover all three queries you need two indexes:

ALTER TABLE ... ADD INDEX (a, b); ALTER TABLE ... ADD INDEX (b)