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目录

CASE 的两种格式： 简单CASE函数 和 CASE搜索函数

同时配合 SUM 以及 COUNT 方法的使用

① SUM函数

② COUNT函数

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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

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**
SELECTorderId,CASE stateWHEN 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**
SELECTorderId,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，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：

SELECTorderId,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”

SELECTorderId,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

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**
SELECTSUM(CASE stateWHEN 1 THEN amtELSE 0 END ) AS sumAmt
FROM t_table
WHEREcore_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**
SELECTSUM(CASE WHEN state = '1' THEN amtELSE 0 END ) AS sumAmt
FROM t_table
WHEREcore_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time**或者**
SELECTSUM(CASE WHEN state = '1' AND t_type = '2' THEN amtELSE 0 END ) AS sumAmt
FROM t_table
WHEREcore_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time**或者**
SELECTSUM(CASE WHEN state IN ('2', '3', '4') AND t_type = '2' THEN amtELSE 0 END ) AS sumAmt
FROM t_table
WHEREcore_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 1ELSE 0 END) AS COUNTAMT**CASE搜索函数**
COUNT(CASE WHEN 条件参数名称 = '参数值' THEN 1ELSE 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 1ELSE 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 1ELSE 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**
SELECTCOUNT(CASE stateWHEN 1 THEN 1ELSE 0 END ) AS countAmt
FROM t_table
WHEREcore_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**
SELECTCOUNT(CASE WHEN state = '1' THEN 1ELSE 0 END ) AS countAmt
FROM t_table
WHEREcore_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time**或者**
SELECTCOUNT(CASE WHEN state = '1' AND t_type = '2' THEN 1ELSE 0 END ) AS countAmt
FROM t_table
WHEREcore_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time**或者**
SELECTCOUNT(CASE WHEN state IN ('2', '3', '4') AND t_type = '2' THEN 1ELSE 0 END ) AS countAmt
FROM t_table
WHEREcore_time >= '2020-07-01 00:00:00'
AND core_time <= '2020-07-20 23:59:59'
GROUP BY core_time

嘟嘟噜~

原文作者：Explorer_S