T-SQL - 習題02_將數據表year|month|amount查詢成year|m1|m2|m3|m4的樣式

• 時間：2017-09-11 整理：byzqy

題目：有個年度統計表，結構如下：

怎么樣把這個表，查詢成這樣一個結果：

這是在面試過程中遇到的一個關於數據庫的題，沒有一點思路，不知它考查到的知識點是什么，如何下手？

網上搜了一下，有哥們兒說這是典型的“行轉列問題”，好像用到“交叉表查詢”，下面看看具體的代碼！

文件：SQLQuery1.sql

/*
環境：Microsoft SQL Server 2012
工具: Microsoft SQL Server Management Studio
數據庫: Interview
數據表: statistic
詳情:
year        month        amount
1991        1            1.1
1991        2            1.2
1991        3            1.3
1991        4            1.4
1992        1            2.1
1992        2            2.2
1992        3            2.3
1992        4            2.4
(char(10))    (int)        (float)
查詢成如下結果:
year    m1     m2     m3     m4
1991    1.1    1.2    1.3    1.4
1992    2.1    2.2    2.3    2.4
*/

--方法1
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
  statistic
GROUP  BY
  year

-->查詢結果:
    year    m1     m2     m3     m4
    1991    1.1    1.2    1.3    1.4
    1992    2.1    2.2    2.3    2.4

--方法2
SELECT 
  year,
  MAX(CASE month WHEN 1 THEN amount ELSE 0 END) m1,
  MAX(CASE month WHEN 2 THEN amount ELSE 0 END) m2, 
  MAX(CASE month WHEN 3 THEN amount ELSE 0 END) m3,
  MAX(CASE month WHEN 4 THEN amount ELSE 0 END) m4
FROM statistic
GROUP BY year

-->查詢結果:
    year    m1     m2     m3     m4
    1991    1.1    1.2    1.3    1.4
    1992    2.1    2.2    2.3    2.4

--方法3
SELECT year, 
(SELECT amount FROM statistic m WHERE month=1 AND m.year=statistic.year) AS m1,
(SELECT amount FROM statistic m WHERE month=2 AND m.year=statistic.year) AS m2,
(SELECT amount FROM statistic m WHERE month=3 AND m.year=statistic.year) AS m3,
(SELECT amount FROM statistic m WHERE month=4 AND m.year=statistic.year) AS m4
FROM statistic GROUP BY year

-->查詢結果:
    year    m1     m2     m3     m4
    1991    1.1    1.2    1.3    1.4
    1992    2.1    2.2    2.3    2.4

運行結果：

代碼分解：

--方法1變形:將代碼中的0改為1,計算SUM,看看會發生什么
SELECT
  year,
  SUM  (  CASE  WHEN  month = 1  THEN  amount ELSE 1 END )  AS  m1,
  SUM  (  CASE  WHEN  month = 2  THEN  amount ELSE 1 END )  AS  m2,
  SUM  (  CASE  WHEN  month = 3  THEN  amount ELSE 1 END )  AS  m3,
  SUM  (  CASE  WHEN  month = 4  THEN  amount ELSE 1 END )  AS  m4
FROM
  statistic
GROUP  BY
  year

-->查詢結果:
    year    m1     m2     m3     m4
    1991    4.1    4.2    4.3    4.4
    1992    5.1    5.2    5.3    5.4

--方法2變形1:將MAX關鍵字換為MIN
SELECT 
  year,
  MIN(CASE month WHEN 1 THEN amount ELSE 0 END) m1,
  MIN(CASE month WHEN 2 THEN amount ELSE 0 END) m2, 
  MIN(CASE month WHEN 3 THEN amount ELSE 0 END) m3,
  MIN(CASE month WHEN 4 THEN amount ELSE 0 END) m4
FROM statistic
GROUP BY year

-->查詢結果:
    year    m1   m2   m3   m4
    1991    0    0    0    0
    1992    0    0    0    0

--方法2變形2:將MAX關鍵字換為MIN,並將0換為1
SELECT 
  year,
  MIN(CASE month WHEN 1 THEN amount ELSE 1 END) m1,
  MIN(CASE month WHEN 2 THEN amount ELSE 1 END) m2, 
  MIN(CASE month WHEN 3 THEN amount ELSE 1 END) m3,
  MIN(CASE month WHEN 4 THEN amount ELSE 1 END) m4
FROM statistic
GROUP BY year

-->查詢結果:
    year    m1   m2   m3   m4
    1991    1    1    1    1
    1992    1    1    1    1

--方法2變形3:將MAX關鍵字換為SUM,並將0換為1
SELECT 
  year,
  SUM(CASE month WHEN 1 THEN amount ELSE 1 END) m1,
  SUM(CASE month WHEN 2 THEN amount ELSE 1 END) m2, 
  SUM(CASE month WHEN 3 THEN amount ELSE 1 END) m3,
  SUM(CASE month WHEN 4 THEN amount ELSE 1 END) m4
FROM statistic
GROUP BY year

-->查詢結果:
    year    m1     m2     m3     m4
    1991    4.1    4.2    4.3    4.4
    1992    5.1    5.2    5.3    5.4

以上整理，如有錯誤之處或有更好的方法，請看到朋友不吝指正，謝謝！

參考文章：

https://zhidao.baidu.com/question/550776347.html

https://zhidao.baidu.com/question/486996750.html

https://www.zybang.com/question/0ac74f3ae058071e80274e23d5e1042a.html