tsql - How do I select the max date from a union of two tables?

Question

SELECT TOP 1 * FROM 
(
    select max(updatedtimestamp) from db1.dbo.policyrepresentationcache with(nolock)
    UNION
    select max(updatedtimestamp) from db2.dbo.policyrepresentationcache with(nolock)
    ) 
ORDER BY updatedtimestamp

I'm running into syntax errors here and im not sure how to set it up.

Answer 1

您只是缺少联合生成的表的别名。我已将其别名为以下。

SELECT TOP 1 a.updatedtimestamp FROM
(
     select max(updatedtimestamp) as updatedtimestamp  
     from db1.dbo.policyrepresentationcache with(nolock)
     UNION
     select max(updatedtimestamp) 
     from db2.dbo.policyrepresentationcache with(nolock)
) a
ORDER BY a.updatedtimestamp

Answer 2

SELECT TOP 1 Temp.* FROM 
(
    select max(updatedtimestamp) from db1.dbo.policyrepresentationcache with(nolock)
    UNION
    select max(updatedtimestamp) from db2.dbo.policyrepresentationcache with(nolock)
    ) AS Temp
ORDER BY Temp.updatedtimestamp