最近遇到一個問題：

把上面表格的資料查詢成：

看到這樣的需求，首先想到的是用case去統計以及 用group by來分組

第一版sql程式碼：

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`

查詢出來的結果有點不如人意：

這麼說明了分組之後只顯示到第一行資料，那麼我們去掉分組看看：

        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

有點像我們想要的了，只是沒有分組以及去掉空值

而且我們可以看出，在分組的情況下m1,m2,m3的值都是一個最大值來的

所以我們可以用一個子查詢來查詢上面的結果集中分組的最大值 最終版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`

最終可以得到我們想要的結果：