What causes the 'subquery returns more than 1 row' error in sql?

This error is raised when a subquery used in an expression returns more than one row, but it's expected to return just one. Example:

SELECT * FROM users WHERE id = (SELECT id FROM users WHERE username LIKE '%John%');
Solution:

-- Ensure the subquery returns a single value
SELECT * FROM users WHERE id IN (SELECT id FROM users WHERE username LIKE '%John%');
If you expect multiple values, use operators like IN, EXISTS, etc.

Beginner's Guide to SQL