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.