Question

the query below throws the error mentioned in the title :

SELECT
   Set_To_Value_ID,
CASE 
   WHEN Set_To_Variable_ID IS NOT NULL THEN Set_To_Variable_ID 
   WHEN Changed_In_SP IS NOT NULL THEN Changed_In_SP
   WHEN Comment_Input IS NOT NULL THEN ( Select Comment FROM Process_Instance WHERE Process_Instance_ID = @Process_Instance_ID )
END AS Manual_Value
FROM Variables
WHERE Some_Irrelevant_Value = @Some_Irrelevant_Value 

I know the error comes from the Comment_Input case because that’s the value that should be returned from there.

Answer 1

When using CASE expressions, all results must have the same data type.

Returns the highest precedence type from the set of types in
result_expressions and the optional else_result_expression.

http://msdn.microsoft.com/en-us/library/ms181765.aspx

I recommend converting your int data types into nvarchar so that SQL doesn’t attempt to convert your nvarchar to int:

CASE 
   WHEN Set_To_Variable_ID IS NOT NULL THEN CAST(Set_To_Variable_ID as nvarchar(20))
   WHEN Changed_In_SP IS NOT NULL THEN THEN CAST(Changed_In_SP as nvarchar(20))
   WHEN Comment_Input IS NOT NULL THEN ( Select Comment FROM Process_Instance WHERE Process_Instance_ID = @Process_Instance_ID )
END AS Manual_Value