Detailed explanation of the use of CASE WHEN function statement under multiple conditions

Table of contents

Two formats of CASE: simple CASE function and CASE search function

At the same time with the use of SUM and COUNT methods

The CASE WHEN function statement implements two formats: simple CASE function and CASE search function.
At the same time with the use of SUM and COUNT methods

Two formats of CASE: simple CASE function and CASE search function

**简单CASE函数**
CASE 条件参数名称
    WHEN 参数值1 THEN '显示值1'
    WHEN 参数值2 THEN '显示值2'
    ...
ELSE '显示其他值' END

**CASE搜索函数**
CASE 
    WHEN 条件参数名称 = '参数值1' THEN '显示值1'
    WHEN 条件参数名称 = '参数值2' THEN '显示值2'
    ...
ELSE '显示其他值' END

Example of two formats:
state: state
order number: orderId

**简单CASE函数**
SELECT
	orderId,
	CASE state
	    WHEN 1 THEN '启动'
	    WHEN 2 THEN '关闭'
	ELSE '未知状态' END AS statusName
FROM t_table

**CASE搜索函数**
SELECT
	orderId,
	CASE 
	    WHEN state = '1' THEN '启动'
	    WHEN state = '2' THEN '关闭'
	ELSE '未知状态' END AS stateName
FROM t_table

These two formats can achieve the same function. However, compared with the CASE search function, the simple CASE function has some limitations in terms of functionality;

For example, write a judgment :

SELECT
	orderId,
	CASE 
	    WHEN state = '1' THEN '启动'
	    WHEN state = '2' THEN '关闭'
	    WHEN state IN ('3', '4') THEN '待解锁'
	ELSE '未知状态' END AS stateName
FROM t_table

If it is multiple, the coincidence of the value in the IN condition of WHEN and the condition of the next WHEN will be ignored , such as the following "closed" state will be ignored, and "closed" will never be obtained

SELECT
	orderId,
	CASE 
	    WHEN state = '1' THEN '启动'
	    WHEN state IN ('2', '3', '4') THEN '待解锁'
	    WHEN state = '2' THEN '关闭'
	ELSE '未知状态' END AS stateName
FROM t_table

At the same time with the use of SUM and COUNT methods

① SUM function

**简单CASE函数**
SUM(CASE 条件参数名称
    	WHEN 参数值 THEN '显示被求和值'
	ELSE 0 END
	) AS SUMAMT


**CASE搜索函数**
SUM(CASE 
    	WHEN 条件参数名称 = '参数值' THEN '显示被求和值'
	ELSE 0 END
	) AS SUMAMT


**或者**	
SUM(CASE 
    	WHEN 
    		条件参数名称1 = '参数值1' AND 条件参数名称2 = '参数值2' ...
    	THEN '显示被求和值'
	ELSE 0 END
	) AS SUMAMT
	

**或者**	
SUM(CASE 
    	WHEN 
    		条件参数名称1 IN ('参数值n') AND 条件参数名称2 = '参数值2' ...
    	THEN '显示被求和值'
	ELSE 0 END
	) AS SUMAMT

Example: Query values based on time conditions, display "summed value" when there is a value, and display "null"
state when there is no value: state
type: t_type
amount: amt

**简单CASE函数**
SELECT
	SUM(CASE state
	    WHEN 1 THEN amt
	ELSE 0 END 
	) AS sumAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**CASE搜索函数**
SELECT
	SUM(CASE 
	    WHEN state = '1' THEN amt
	ELSE 0 END 
	) AS sumAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**或者**
SELECT
	SUM(CASE 
	    WHEN state = '1' AND t_type = '2' THEN amt
	ELSE 0 END 
	) AS sumAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**或者**
SELECT
	SUM(CASE 
	    WHEN state IN ('2', '3', '4') AND t_type = '2' THEN amt
	ELSE 0 END 
	) AS sumAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time

② COUNT function

**简单CASE函数**
COUNT(CASE 条件参数名称
    	WHEN 参数值 THEN 1
	ELSE 0 END
	) AS COUNTAMT


**CASE搜索函数**
COUNT(CASE 
    	WHEN 条件参数名称 = '参数值' THEN 1
	ELSE 0 END
	) AS COUNTAMT


**或者**	
COUNT(CASE 
    	WHEN 
    		条件参数名称1 = '参数值1' AND 条件参数名称2 = '参数值2' ...
    	THEN 1
	ELSE 0 END
	) AS COUNTAMT

	
**或者**	
COUNT(CASE 
    	WHEN 
    		条件参数名称1 IN ('参数值n') AND 条件参数名称2 = '参数值2' ...
    	THEN 1
	ELSE 0 END
	) AS COUNTAMT

Example: Query the value according to the time condition, if there is a value, it will display "total value", if there is no value, it will display "0"
state: state
type: t_type
amount: amt

**简单CASE函数**
SELECT
	COUNT(CASE state
	    WHEN 1 THEN 1
	ELSE 0 END 
	) AS countAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**CASE搜索函数**
SELECT
	COUNT(CASE 
	    WHEN state = '1' THEN 1
	ELSE 0 END 
	) AS countAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**或者**
SELECT
	COUNT(CASE 
	    WHEN state = '1' AND t_type = '2' THEN 1
	ELSE 0 END 
	) AS countAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time


**或者**
SELECT
	COUNT(CASE 
	    WHEN state IN ('2', '3', '4') AND t_type = '2' THEN 1
	ELSE 0 END 
	) AS countAmt
FROM 
	t_table
WHERE
	core_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time

Dudulu~

Original author: Explorer_S