巧用 CASE WHEN THEN END

巧用 CASE WHEN THEN END

一.CASE WHEN THEN END結構有兩種書寫方式
1. 簡單CASE WHEN函數：

CASE SCORE WHEN 'A' THEN '優' ELSE '不及格' END
CASE SCORE WHEN 'B' THEN '良' ELSE '不及格' END
CASE SCORE WHEN 'C' THEN '中' ELSE '不及格' END

2.CASE WHEN條件表達式函數實現：

CASE WHEN SCORE = 'A' THEN '優'
     WHEN SCORE = 'B' THEN '良'
     WHEN SCORE = 'C' THEN '中' ELSE '不及格' END

簡單CASE WHEN函數只能應對一些簡單的業務場景，而CASE WHEN條件表達式的寫法則更加靈活。
二.幾種常用場景
場景1： 有分數score，score<60返回不及格，score>=60返回及格，score>=80返回優秀

SELECT
    STUDENT_NAME,
    (CASE WHEN score < 60 THEN '不及格'
        WHEN score >= 60 AND score < 80 THEN '及格'
        WHEN score >= 80 THEN '優秀'
        ELSE '異常' END) AS REMARK
FROM
    TABLE

注意： 如果你想判斷score是否null的情況，WHEN score = null THEN ‘缺席考試’，這是一種錯誤的寫法，正確的寫法應爲：
CASE WHEN score IS NULL THEN ‘缺席考試’ ELSE ‘正常’ END

場景2： 現老師要統計班中，有多少男同學，多少女同學，並統計男同學中有幾人及格，女同學中有幾人及格，要求用一個SQL輸出結果。

a.表結構如下： 其中STU_SEX字段，0表示男生，1表示女生。

b.SQL

SELECT 
	SUM (CASE WHEN STU_SEX = 0 THEN 1 ELSE 0 END) AS MALE_COUNT,
	SUM (CASE WHEN STU_SEX = 1 THEN 1 ELSE 0 END) AS FEMALE_COUNT,
	SUM (CASE WHEN STU_SCORE >= 60 AND STU_SEX = 0 THEN 1 ELSE 0 END) AS MALE_PASS,
	SUM (CASE WHEN STU_SCORE >= 60 AND STU_SEX = 1 THEN 1 ELSE 0 END) AS FEMALE_PASS
FROM 
	THTF_STUDENTS

c.輸出結果：

場景3： 經典行轉列，並配合聚合函數做統計
現要求統計各個城市，總共使用了多少水耗、電耗、熱耗，使用一條SQL語句輸出結果
a.有能耗表如下： 其中，E_TYPE表示能耗類型，0表示水耗，1表示電耗，2表示熱耗

b.SQL

SELECT 
	E_CODE,
	SUM(CASE WHEN E_TYPE = 0 THEN E_VALUE ELSE 0 END) AS WATER_ENERGY,--水耗
	SUM(CASE WHEN E_TYPE = 1 THEN E_VALUE ELSE 0 END) AS ELE_ENERGY,--電耗
	SUM(CASE WHEN E_TYPE = 2 THEN E_VALUE ELSE 0 END) AS HEAT_ENERGY--熱耗
FROM 
	THTF_ENERGY_TEST
GROUP BY
	E_CODE

c.輸出結果：

場景4： CASE WHEN中使用子查詢
根據城市用電量多少，計算用電成本。假設電能耗單價分爲三檔，根據不同的能耗值，使用相應價格計算成本。
a.價格表如下：

b.SQL

CASE WHEN energy <= (SELECT P_LIMIT FROM TABLE_PRICE WHERE P_LEVEL = 0) THEN (SELECT P_PRICE FROM TABLE_PRICE WHERE P_LEVEL = 0)
    WHEN energy > (SELECT P_LIMIT FROM TABLE_PRICE WHERE P_LEVEL = 0) AND energy <= (SELECT P_LIMIT FROM TABLE_PRICE WHERE P_LEVEL = 1) THEN (SELECT P_PRICE FROM TABLE_PRICE WHERE P_LEVEL = 1)
    WHEN energy > (SELECT P_LIMIT FROM TABLE_PRICE WHERE P_LEVEL = 1) AND energy <= (SELECT P_LIMIT FROM TABLE_PRICE WHERE P_LEVEL = 2) THEN (SELECT P_PRICE FROM TABLE_PRICE WHERE P_LEVEL = 2)

場景5： 結合max聚合函數
a.SQL

select 
	max(case when column_name='shui_hao' then comments else '' end ) as shui_hao_comments,
	max(case when column_name='dian_hao' then comments else '' end ) as dian_hao_comments,
	max(case when column_name='re_hao' then comments else '' end ) as re_hao_comments,
	max(case when column_name='yan_hao' then comments else '' end ) as yan_hao_comments,
	max(case when column_name='other1' then comments else '' end ) as other1_comments,
	max(case when column_name='other2' then comments else '' end ) as other2_comments,
	max(case when column_name='other3' then comments else '' end ) as other3_comments
from user_col_comments
where table_name='thtf_table_year_2018'

b.結果輸出：

根據工作中的實際的工作場景，巧用case when then end ，可以達到事半功倍的效果！