MySQL: case when 用法

简单函数：

CASE [col_name] WHEN [value1] THEN [result1]…ELSE [default] END；枚举这个字段所有可能的值。

SELECT
    DATE_FORMAT( paytime, ‘%Y-%m-%d-%H‘ ) payhours,
    state,
    CASE state
            WHEN ‘1‘ THEN ‘交易成功‘ 
            WHEN ‘2‘ THEN ‘交易失败‘ 
            WHEN ‘0‘ THEN ‘待支付‘ 
            ELSE ‘无‘ 
    END ‘状态名称‘
FROM
    payorder 
WHERE
    DATE_FORMAT( paytime, ‘%Y-%m-%d‘ ) = ‘2020-05-26‘ 
GROUP BY
    payhours ASC; 

查询结果：

 搜索函数：

CASE WHEN [expr] THEN [result1]…ELSE [default] END；搜索函数可以写判断，并且搜索函数只会返回第一个符合条件的值，其他case被忽略.即：在when后面的表达式中写出要搜索的判断条件，然后在THEN后面写上判断结果。

SELECT
    DATE_FORMAT( paytime, ‘%Y-%m-%d-%H‘ ) payhours,
    CASE 
            WHEN paytime <=‘2020-05-26 09:00:00‘ THEN ‘早上‘ 
            WHEN paytime <=‘2020-05-26 12:59:59‘ THEN ‘上午‘  
            ELSE ‘下午‘ 
    END ‘时间段‘
FROM
    payorder 
WHERE
    DATE_FORMAT( paytime, ‘%Y-%m-%d‘ ) = ‘2020-05-26‘ 
GROUP BY
    payhours ASC; 

查询结果：

 聚合函数 sum 配合 case when 的简单函数实现行转列

SELECT
    DATE_FORMAT( paytime, ‘%Y-%m-%d-%H‘ ) payhours,
    sum( CASE state WHEN ‘1‘ THEN 1 ELSE 0 END ) AS state1,
    sum( CASE state WHEN ‘2‘ THEN 1 ELSE 0 END ) AS state2,
    sum( CASE state WHEN ‘0‘ THEN 1 ELSE 0 END ) AS state0 ,
    count(payorderid) AS countsum
FROM
    payorder 
WHERE
    DATE_FORMAT( paytime, ‘%Y-%m-%d‘ ) = ‘2020-05-26‘ 
GROUP BY
    payhours ASC;

MySQL: case when 用法

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