巧用 CASE WHEN THEN END

巧用 CASE WHEN THEN END

一.CASE WHEN THEN END结构有两种书写方式
1. 简单CASE WHEN函数：

CASE SCORE WHEN 'A' THEN '优' 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条件表达式函数实现：

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

简单CASE WHEN函数只能应对一些简单的业务场景，而CASE WHEN条件表达式的写法则更加灵活。
二.几种常用场景
场景1： 有分数score，score<60返回不及格，score>=60返回及格，score>=80返回优秀

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

注意： 如果你想判断score是否null的情况，WHEN score = null THEN ‘缺席考试’，这是一种错误的写法，正确的写法应为：
CASE WHEN score IS NULL THEN ‘缺席考试’ ELSE ‘正常’ END

场景2： 现老师要统计班中，有多少男同学，多少女同学，并统计男同学中有几人及格，女同学中有几人及格，要求用一个SQL输出结果。

a.表结构如下： 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.输出结果：

场景3： 经典行转列，并配合聚合函数做统计
现要求统计各个城市，总共使用了多少水耗、电耗、热耗，使用一条SQL语句输出结果
a.有能耗表如下： 其中，E_TYPE表示能耗类型，0表示水耗，1表示电耗，2表示热耗

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,--电耗
	SUM(CASE WHEN E_TYPE = 2 THEN E_VALUE ELSE 0 END) AS HEAT_ENERGY--热耗
FROM 
	THTF_ENERGY_TEST
GROUP BY
	E_CODE

c.输出结果：

场景4： CASE WHEN中使用子查询
根据城市用电量多少，计算用电成本。假设电能耗单价分为三档，根据不同的能耗值，使用相应价格计算成本。
a.价格表如下：

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)

场景5： 结合max聚合函数
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.结果输出：

根据工作中的实际的工作场景，巧用case when then end ，可以达到事半功倍的效果！