SQL之CASE WHEN函数语句多条件下使用详解

针对CASE WHEN函数语句，实现简单CASE函数和CASE搜索函数两种格式。
同时配合 SUM以及COUNT方法的使用

1、CASE 的两种格式： 简单CASE函数和CASE搜索函数

**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

两种格式示例：
状态：state
订单号：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

这两种格式式，可以实现相同的功能。但是简单CASE函数和CASE搜索函数相比，功能方面会有些限制，比如写判断式：

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

若是多重时，WHEN 的 IN 条件中的值和下一个 WHEN 的条件重合会被忽略，如下面的 “关闭” 状态会被忽略，永远无法得到 “关闭”

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

2、同时配合 SUM以及COUNT方法的使用

（1）SUM函数

**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

示例： 根据时间条件查询值，有值时显示 “被求和值”， 没有值时，显示 “null”
状态：state
类型：t_type
金额：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函数

**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

示例： 根据时间条件查询值，有值时显示 “总条数值”， 没有值时，显示 “0”
状态：state
类型：t_type
金额：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