When subquery is used, the database server (actually the query optimizer) may need to perform additional steps, such as sorting, before the results from the subquery are used. If a query that contains subqueries can be rewritten as a join, you should use join rather than subqueries. This is because using join typically allows the query optimizer to retrieve data in the most efficient way.
In other words, The optimizer is more mature for MySQL for joins than for subqueries, so in many cases a statement that uses a subquery can be executed more efficiently if you rewrite it as a join.
