SQL:如何向查询添加子查询

SELECT 列名
FROM 表名
WHERE 列名 操作符 (SELECT 列名 FROM 表名 WHERE 条件);

SELECT 列名
FROM 表名
WHERE 列名 操作符 (SELECT 列名 FROM 表名 WHERE 条件);

SELECT *
FROM orders
WHERE total_amount > (SELECT AVG(total_amount) FROM orders);

SELECT 列名
FROM (SELECT 列名 FROM 表名 WHERE 条件) AS 子查询表名;

SELECT department, COUNT(*) AS employee_count
FROM (SELECT department FROM employees) AS subquery
GROUP BY department;

SELECT 列名, (SELECT 列名 FROM 表名 WHERE 条件) AS 列别名
FROM 表名;

SELECT employee_id, (SELECT SUM(amount) FROM sales WHERE employee_id = employees.employee_id) AS total_sales
FROM employees;