如何在CASE中执行SELECT语句

SELECT FullName FROM TableA
union 
SELECT FirstName FROM  TableB 
where not exists 
  (SELECT FullName FROM TableA)

Working Fiddle

我想这就是你要寻找的:

IF EXISTS (SELECT 1 FROM TableA) 
    SELECT FullName AS Name, TableA.Field1 AS FinalField1, 
           TableA.Field2 AS FinalField2... 
    FROM TableA
ELSE
    SELECT FirstName AS Name, TableB.Field1 AS FinalField1, 
           TableB.Field2 AS FinalField2... 
    FROM TableB