COALESCE(x, y)
CREATE TABLE purchase_log (
purchase_id varchar(255)
, amount integer
, coupon integer
);
INSERT INTO purchase_log
VALUES
('10001', 3280, NULL)
, ('10002', 4650, 500)
, ('10003', 3870, NULL)
;
SELECT
purchase_id
, amount
, coupon
, amount - coupon AS 할인가격1
, amount - COALESCE(coupon, 0) AS 할인가격2
FROM purchase_log;
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https://brownbears.tistory.com/539