Subqueries behind exists, exists only care about there, returns a Boolean type
Case #: Query department employees have
SELECT department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
WHERE d.`department_id`=e.`department_id`
);
or
SELECT department_name
FROM departments
WHERE department_id IN(
SELECT department_id
FROM employees
);
# Information inquiry god no girlfriend
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT b.boyfriend_id
FROM beauty b
WHERE b.`boyfriend_id`=bo.`id`
);
or
SELECT bo.*
FROM boys bo
WHERE bo.`id` NOT IN(
SELECT boyfriend_id
FROM beauty b
);