标签：需求 table 代码 情况 mount div group style line
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 |
标签：需求 table 代码 情况 mount div group style line
原文地址：https://www.cnblogs.com/TimLiuDream/p/9902191.html
