mysql中case的一个例子

最近遇到一个问题：

year	amount	num
1991	1	1.1
1991	2	1.2
1991	3	1.3
1992	1	2.1
1992	2	2.2
1992	3	3.3

把上面表格的数据查询成：

year	m1	m2	m3
1991	1.1	1.2	1.3
1992	2.1	2.2	2.3

看到这样的需求，首先想到的是用case去统计以及 用group by来分组

第一版sql代码：

SELECT
    `year`,
    (CASE WHEN amount = 1 THEN num END) AS n1,
    (CASE WHEN amount = 2 THEN num END) AS n2,
    (CASE WHEN amount = 3 THEN num END) AS n3
FROM
    test
GROUP BY
    `year`

查询出来的结果有点不如人意：

year	n1	n2	n3
1991	1.1
1992	2.1

这么说明了分组之后只显示到第一行数据，那么我们去掉分组看看：

        SELECT
            `year`,
            (CASE WHEN amount = 1 THEN num END) AS n1,
            (CASE WHEN amount = 2 THEN num END) AS n2,
            (CASE WHEN amount = 3 THEN num END) AS n3
        FROM
            test

year	n1	n2	n3
1991	1.1
1991		1.2
1991			1.3
1992	2.1
1992		2.2
1992			2.3

有点像我们想要的了，只是没有分组以及去掉空值

而且我们可以看出，在分组的情况下m1,m2,m3的值都是一个最大值来的

所以我们可以用一个子查询来查询上面的结果集中分组的最大值 最终版sql：

SELECT
    `year`,
    MAX(n1) AS m1,
    MAX(n2) AS m2,
    MAX(n3) AS m3
FROM
    (
        SELECT
            `year`,
            (CASE WHEN amount = 1 THEN num END) AS n1,
            (CASE WHEN amount = 2 THEN num END) AS n2,
            (CASE WHEN amount = 3 THEN num END) AS n3
        FROM
            test
    ) AS a
GROUP BY
    `year`

最终可以得到我们想要的结果：

year	m1	m2	m3
1991	1.1	1.2	1.3
1992	2.1	2.2	2.3