Sub Query

1) WHERE 절에서 사용

SELECT * FROM tbl_cake WHERE price >= (SELECT AVG(price) FROM tbl_cake);

2) HAVING 절에서 사용

SELECT code, MIN(arrival) FROM tbl_stock GROUP BY code
            HAVING MIN(arrival) < (SELECT AVG(shipment) FROM tbl_stock);

3) FROM 절에서 사용

SELECT MIN(price)
            FROM (SELECT * FROM tbl_cake WHERE price >= 2500) AS c_price;

* 응용

INSERT INTO tbl_advance SELECT * FROM tbl_results
    WHERE point1 + point2  > (SELECT AVG(point1 + point2) FROM tbl_results);

UPDATE tbl_allowance SET total = (overtime + travel)
     WHERE overtime + travel < (SELECT MAX(overtitme) FROM tbl_allowance);

DELETE FROM tbl_allowance WHERE travel > (SELECT AVG(travel)
            FROM tbl_allowance WHERE overtime >= 40000);