Example
Query
SELECT * FROM stack WHERE username = "admin" AND password = "admin";
Result
+------+----------+----------+
| id | username | password |
+------+----------+----------+
| 1 | admin | admin |
+------+----------+----------+
1 row in set (0.00 sec)
Query with a nested SELECT in the WHERE clause
The WHERE clause can contain any valid SELECT statement to write more complex queries. This is a 'nested' query
Query
Nested queries are usually used to return single atomic values from queries for comparisons.
SELECT title FROM books WHERE author_id = (SELECT id FROM authors WHERE last_name = 'Bar' AND first_name = 'Foo');
Selects all usernames with no email address
SELECT * FROM stack WHERE username IN (SELECT username FROM signups WHERE email IS NULL);
Disclaimer: Consider using joins for performance improvements when comparing a whole result set.
