在结果集上使用 case 表达式？

Question

I'm trying to write a SQL query with case and the conditions of the case is dependent on whether any records are found on a sub-query.

Select 
  Case When (Another Select statement which may return results) 'True'
  Else 'False'
  End As Has_Results
From TBL_ABC

Answer 1

You want an EXISTS condition in a CASE expression:

Select Case 
    When Exists (
        Another Select statement which may return results
    ) 
    Then 'True'
    Else 'False'
End As Has_Results
From TBL_ABC

Answer 2

You can use EXISTS:

SELECT
CASE
    WHEN exists(other query) THEN 'True'  --Subquery has record   
    ELSE 'False'                          --Subquery has not record     
END;