The MySQL query optimizer has different strategies available to evaluate subqueries:
1、 For a subquery used with an IN, = ANY, or EXISTS predicate, the optimizer has these choices:
• Semijoin
• Materialization
• EXISTS strategy

2、 For a subquery used with a NOT IN, <> ALL or NOT EXISTS predicate, the optimizer has these
choices:
1669Optimizing Subqueries, Derived Tables, View References, and Common Table Expressions
• Materialization
• EXISTS strategy

3、For a derived table, the optimizer has these choices (which also apply to view references and common
table expressions):
• Merge the derived table into the outer query block
• Materialize the derived table to an internal temporary table
The following discussion provides more information about the preceding optimization strategies.

Note
A limitation on UPDATE and DELETE statements that use a subquery to modifya single table is that the optimizer does not use semijoin or materialization subquery optimizations. As a workaround, try rewriting them as multiple-table UPDATE and DELETE statements that use a join rather than a subquery.