SQL 学习2

把一个按月统计表，变成按年统计

year	month	amount
1991	1	1.1
1991	2	1.2
1991	3	1.3
1991	4	1.4
1992	1	1.1
1992	2	1.2
1992	3	1.3
1992	4	1.4

用sql生成下面的表：

year	m1	m2	m3	m4
1991	1.1	1.2	1.3	1.4
1992	1.1	1.2	1.3	1.4

SELECT 
    year,
    SUM(CASE
        WHEN month = 1 THEN amount
        ELSE 0
    END) AS m1,
    SUM(CASE
        WHEN month = 2 THEN amount
        ELSE 0
    END) AS m2,
    SUM(CASE
        WHEN month = 3 THEN amount
        ELSE 0
    END) AS m3,
    SUM(CASE
        WHEN month = 4 THEN amount
        ELSE 0
    END) AS m4
FROM
    t1
GROUP BY year;

有两个表A 和B ，均有key 和value 两个字段，如果B 的key 在A 中也有，就把B 的value 换为A 中对应的value

这道题的SQL 语句怎么写？

UPDATE b 
SET 
    b.value = (SELECT 
            a.value
        FROM
            a
        WHERE
            a.key = b.key)
WHERE
    b.key IN (SELECT 
            a.key
        FROM
            a);