Join vs. sub-query

Question

I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query, and I hate it; I don\'t know why.

Answer 1

Subqueries are generally used to return a single row as an atomic value, though they may be used to compare values against multiple rows with the IN keyword. They are allowed at nearly any meaningful point in a SQL statement, including the target list, the WHERE clause, and so on. A simple sub-query could be used as a search condition. For example, between a pair of tables:

   SELECT title FROM books WHERE author_id = (SELECT id FROM authors WHERE last_name = 'Bar' AND first_name = 'Foo');

Note that using a normal value operator on the results of a sub-query requires that only one field must be returned. If you're interested in checking for the existence of a single value within a set of other values, use IN:

   SELECT title FROM books WHERE author_id IN (SELECT id FROM authors WHERE last_name ~ '^[A-E]');

This is obviously different from say a LEFT-JOIN where you just want to join stuff from table A and B even if the join-condition doesn't find any matching record in table B, etc.

If you're just worried about speed you'll have to check with your database and write a good query and see if there's any significant difference in performance.