mysql怎样调用子查询

-- 查询工资最高的员工
SELECT name, salary
FROM employees
WHERE salary = (SELECT MAX(salary) FROM employees);

-- 查询每个部门最高工资的员工
SELECT department_id, name, salary
FROM employees e1
WHERE salary IN (SELECT MAX(salary) FROM employees e2 WHERE e1.department_id = e2.department_id);

-- 查询工资和职位都匹配的员工
SELECT *
FROM employees
WHERE (salary, job_title) = (SELECT salary, job_title FROM employees WHERE employee_id = 1);

-- 查询每个部门的员工信息
SELECT d.department_name, e.employee_id, e.name, e.salary
FROM departments d
JOIN employees e ON d.department_id = e.department_id;

EXPLAIN SELECT name, salary
FROM employees
WHERE salary = (SELECT MAX(salary) FROM employees);

SELECT name, salary
FROM employees
WHERE salary = (SELECT MAX(salary) FROM employees)
   OR (SELECT MAX(salary) FROM employees) IS NULL;