MYSQL - Difference between IN and EXIST

Question

MySql question:

What is the difference between [NOT] IN and [NOT] EXIST when doing subqueries in MySql.

Answer 1

They work very differently:

• EXISTS takes a single argument which should be a subquery (derived table) and checks if there is at least one row returned by the subquery.
• IN takes two arguments, the first of which should be a single value (or a tuple), and the second of which is a subquery or a tuple and checks if the first value is contained in second.

However both can be used to check if a row in table A has a matching row in table B. Unless you are careful and know what you are doing I would stay clear of IN in MySQL as it often gives much poorer performance on more complex queries. Use NOT EXISTS or a LEFT JOIN ... WHERE ... IS NULL.