在 PostgreSQL 中， COALESCE函数返回第一个非空参数。它通常与 SELECT 语句一起使用以有效处理空值。

CREATE TABLE items (
    ID serial PRIMARY KEY,
    product VARCHAR (100) NOT NULL,
    price NUMERIC NOT NULL,
    discount NUMERIC
);

INSERT INTO items (product, price, discount)
VALUES
    ('A', 1000, 10),
    ('B', 1500, 20),
    ('C', 800, 5),
    ('D', 500, NULL);

   select product,(price-discount) as net_price from items

select product,(price-coalesce(discount,0)) as net_price from items