Question

Given 3 possible values for X (A, B, C), is it faster to do:

1. WHERE (X = 'B' OR X = 'C'), or
2. WHERE X != 'A'

Or does it depend? If so, then what does it depend on?

Answer 1

• Option 1:

  WHERE (X = 'B' OR X = 'C')
  

  and

  WHERE X IN ('B', 'C')
  

  are equivalent and may use an index on (X).

• Option 2:

  WHERE X != 'A'
  

  will not use an index on (X). See a comment by Henrik Grubbström at the MySQL docs, How MySQL Optimizes WHERE Clauses page:

Indexes are ignored for the <> operator:

So, if the use of index makes the query faster (for example, if 99% of the table has X = 'A'), use the first option.

Note: The != operator is a synonym (in MySQL) of the SQL-standard <> inequality operator.