mysql的子查询如何连接

SELECT column1, column2, ...
FROM table1
WHERE column_name operator (SELECT STATEMENT);

SELECT *
FROM employees e
WHERE e.salary > (SELECT AVG(salary) FROM employees WHERE department_id = e.department_id);

SELECT column1, column2, ...
FROM (SELECT STATEMENT) AS subquery_alias
WHERE condition;

SELECT department_id, avg_salary
FROM (
    SELECT department_id, AVG(salary) AS avg_salary
    FROM employees
    GROUP BY department_id
) AS dept_avg_salaries
WHERE avg_salary > 5000;

SELECT column1, column2, (SELECT STATEMENT) AS alias
FROM table;

SELECT e.employee_name, (
    SELECT m.employee_name
    FROM employees m
    WHERE m.employee_id = e.manager_id
) AS manager_name
FROM employees e;

SELECT column1, column2, ...
FROM table
WHERE EXISTS (SELECT STATEMENT);

SELECT customer_id
FROM customers c
WHERE EXISTS (
    SELECT order_id
    FROM orders o
    WHERE o.customer_id = c.customer_id
);