从具有两个不同命名列的两个不同表中获取最近使用的日期

Question

我有两张表table 1和table 2，

table 1的列名为sno和useddate table 2的列名为sno和recentlyuseddate

我想比较这两个列useddate和recentlyusedate并获取最近使用的日期。

这些表可能有很多条目，但在比较这些表后我只需要一个ENTRY（最近的）日期。

Answer 1

SELECT MAX(useddate) AS mostrecent
FROM 
(SELECT useddate FROM Table1
UNION ALL
SELECT recentlyuseddate AS useddate FROM Table2) TheUnion

Answer 2

您可以使用联盟

 SELECT MAX(col)
 FROM (SELECT col FROM TABLE_1
    UNION ALL
    SELECT col FROM TABLE_2)

Answer 3

如果你有两个表中两个日期的索引，我会选择：

select top 1 sno, useddate, which
from ((select top 1 sno, useddate, 'table1' as which from table1 order by useddate desc) union all
      (select top 1 sno, recentlyuseddate , 'table2' as which from table2 order by recentlyuseddate desc)
     )
order by useddate desc;

此版本还会告诉您日期来自哪个表格。