如何从最大日期子查询列表中选择最大日期

Question

我需要从不同日期的一堆日期中获取MAX日期。我有一个查询返回6个日期。我需要以某种方式获得该查询的最大值。这是我的作品，给我6个不同的MAX日期第一个查询：

SELECT 
     MAX(mi.last_updated_dts), 
     (SELECT MAX(lt.created_dts) FROM live_training as lt WHERE lt.me_id = 1156), 
     (SELECT MAX(gm.created_dts) FROM group_member as gm WHERE gm.me_id = 1156), 
     (SELECT MAX(clm.created_dts) FROM contact_list_member as clm WHERE clm.me_id = 1156), 
     (SELECT MAX(mc.created_dts) FROM member_case as mc WHERE mc.me_id = 1156), 
     (SELECT MAX(mcc.created_dts) FROM member_case_comment as mcc INNER JOIN member_case as mc ON (mcc.member_case_id = mc.id) WHERE mc.me_id = 1156) 
    FROM 
     member_info AS mi 
    WHERE mi.id = 1276 

这里是什么样的，我试图做的，但它不工作：

SELECT MAX(
      SELECT 
        MAX(mi.last_updated_dts), 
        (SELECT MAX(lt.created_dts) FROM live_training as lt WHERE lt.me_id = 1156), 
        (SELECT MAX(gm.created_dts) FROM group_member as gm WHERE gm.me_id = 1156), 
        (SELECT MAX(clm.created_dts) FROM contact_list_member as clm WHERE clm.me_id = 1156), 
        (SELECT MAX(mc.created_dts) FROM member_case as mc WHERE mc.me_id = 1156), 
        (SELECT MAX(mcc.created_dts) FROM member_case_comment as mcc INNER JOIN member_case as mc ON (mcc.member_case_id = mc.id) WHERE mc.me_id =1156) 
      FROM 
        member_info AS mi 
      WHERE mi.id = 1276 
     ) as theMostMaxDate 

我不不知道应该从何声明中输入什么，或者甚至可能。谢谢你的帮助！

Answer 1

一个解决方案可能是重写查询以在派生表中使用union all并从中获取最大值，尽管这只会得到一个日期而没有关于其来源的信息（但如果这很重要，您可以添加一列与一个文字值指示查询什么给你最大日期）：

SELECT MAX(max_date) 
FROM (
    SELECT MAX(mi.last_updated_dts) AS max_date FROM member_info AS mi WHERE mi.id = 1276 
    UNION ALL 
    SELECT MAX(lt.created_dts) FROM live_training as lt WHERE lt.me_id = 1156 
    UNION ALL 
    SELECT MAX(gm.created_dts) FROM group_member as gm WHERE gm.me_id = 1156 
    UNION ALL 
    SELECT MAX(clm.created_dts) FROM contact_list_member as clm WHERE clm.me_id = 1156 
    UNION ALL 
    SELECT MAX(mc.created_dts) FROM member_case as mc WHERE mc.me_id = 1156 
    UNION ALL 
    SELECT MAX(mcc.created_dts) FROM member_case_comment as mcc 
    INNER JOIN member_case as mc ON (mcc.member_case_id = mc.id) WHERE mc.me_id = 1156 
) t 

Answer 2

MAX（）是一组由功能和工作在一个单一的字段或表达式。使用greates()函数可以在多个字段或表达式中工作。

SELECT 
    GREATEST(MAX(mi.last_updated_dts), 
    (SELECT MAX(lt.created_dts) FROM live_training as lt WHERE lt.me_id = 1156), 
    (SELECT MAX(gm.created_dts) FROM group_member as gm WHERE gm.me_id = 1156), 
    (SELECT MAX(clm.created_dts) FROM contact_list_member as clm WHERE clm.me_id = 1156), 
    (SELECT MAX(mc.created_dts) FROM member_case as mc WHERE mc.me_id = 1156), 
    (SELECT MAX(mcc.created_dts) FROM member_case_comment as mcc INNER JOIN member_case as mc ON (mcc.member_case_id = mc.id) WHERE mc.me_id = 1156)) as maxdate 
FROM 
    member_info AS mi 
WHERE mi.id = 1276 

另一种解决方案是将独立查询与union结合起来，并且可以使用max（）从结果集中选择总体最大值。

Answer 3

@jpw对此问题有正确的解决方案。我只是想指出的是，你可以不用子查询：

SELECT MAX(mi.last_updated_dts) AS max_date FROM member_info AS mi WHERE mi.id = 1276 
UNION ALL 
SELECT MAX(lt.created_dts) FROM live_training as lt WHERE lt.me_id = 1156 
UNION ALL 
SELECT MAX(gm.created_dts) FROM group_member as gm WHERE gm.me_id = 1156 
UNION ALL 
SELECT MAX(clm.created_dts) FROM contact_list_member as clm WHERE clm.me_id = 1156 
UNION ALL 
SELECT MAX(mc.created_dts) FROM member_case as mc WHERE mc.me_id = 1156 
UNION ALL 
SELECT MAX(mcc.created_dts) FROM member_case_comment as mcc 
INNER JOIN member_case as mc ON (mcc.member_case_id = mc.id) WHERE mc.me_id = 1156 
ORDER BY max_date DESC 
LIMIT 1;