Mysql 使用联接表SQL中的if条件确定列数据

我有一个很好用的查询，我想要帮助的是如何合并一个条件来确定给定列上显示的值。也就是说，如果表的一列有一个值，比如“COA授权”，那么添加的列日期应该显示日期，否则该列应该显示null

这是我目前的疑问

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , t.date_added (//should show the value of date_added else NULL based on a condition for this column),
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

尝试一下：

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , 
IFF(nameOfColumnToTest = 'Authorization of COA', t.date_added, null) AS DateAdded 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

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在mysql和ms sql中，您可以使用大小写表达式（；）有条件地为字段赋值：

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , 
    CASE WHEN Column_To_Test = 'Authorization of COA' THEN t.date_added
        ELSE null
    END AS date_added 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'
    AND r.created_at BETWEEN '$start' AND '$end'
    AND a_s.department_id = '$dept'
    GROUP BY r.request_id
    ORDER BY `r`.`created_at` DESC "

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这样，解决方案在数据库之间更具可移植性。Mysql提供if（）函数，ms sql iif（）来实现上述功能，但这些都是特定于产品的解决方案。

我相信，在这种情况下，一个简单的案例说明就足够了

SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, 
CONCAT(u.fname,' ',u.lname) 'analyst', 
CASE WHEN ConditionColumn = 'Authorization of COA' THEN t.date_added 
ELSE NULL END AS 'date_added' 
    FROM request r
    LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
    LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
    LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
    WHERE r.client_id='$cid'

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尝试使用Case语句，如

当（条件1和条件2或…条件N）时，则（值1）其他（值1）结束

这里，条件\u 1可以是列记录值及其列记录值的组合

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和Value\u 1可以是单值[Ex:Null或“某个字符串”]或嵌套sql语句返回的值，如[Ex:（select count（*）from…）

在mysql use if（）函数或case表达式中标记您知道的所有RDBMS，或者在ms sql用例表达式中仅保留一个：）。