Mysql's subquery
For the subquery after exists, exists only cares about whether there is one, and returns a Boolean type.
# case: query the name of the department with employees
SELECT department_nameFROM departments dWHERE EXISTS (SELECT * FROM employees e WHERE d.`department _ id` = e.`department _ id`)
Or
SELECT department_nameFROM departmentsWHERE department_id IN (SELECT department_id FROM employees)
# query the information of male gods who do not have girlfriends
SELECT bo.*FROM boys boWHERE NOT EXISTS (SELECT b.boyfriend_id FROM beauty b WHERE b.`boyfriend _ id` = bo.`id`)
Or
SELECT bo.*FROM boys boWHERE bo.`id`NOT IN (SELECT boyfriend_id FROM beauty b)
