使用另一个表中列的平均值

Question

我有两张桌子：

CARMAKES
id, name, avg_sold_for

SALES
carmake_id, price

我想要做的是使用单个SQL语句执行以下操作： 对于CARMAKES中的所有行，avg_sold_for字段应显示SALES中所有价格字段的平均值，其中carmake_id与CARMAKES中的给定ID匹配。

以示例：

CARMAKES
id, name, avg_sold_for
0, "Toyota", 0
1, "Nissan", 0

SALES
carmake_id, price
0, 10
1, 20
0, 30
0, 20

我想将CARMAKES表更新为

CARMAKES
id, name, avg_sold_for
0, "Toyota", 20
1, "Nissan", 20

Answer 1

试试这个：

UPDATE CARMAKES
SET avg_sold_for = (SELECT SUM(price) / COUNT(carmake_id) 
                    FROM SALES WHERE CARMAKES.id = SALES.carmake_id)

或

UPDATE CARMAKES C 
INNER JOIN (SELECT carmake_id, (SUM(price) / COUNT(carmake_id)) avg_sold_for
            FROM SALES GROUP BY carmake_id) A ON C.id = A.carmake_id
SET C.avg_sold_for = A.avg_sold_for