Conditional Inner Join with two Functions - sql

Question

I have two functions. I need to decide whether to join with these two based on a BIT Value.
ON APEL.data_Period_dataination_Lookup_ID = CCEL.data_Period_dataination_Lookup_ID
INNER JOIN markcommon.GetPredecessordatadataIds() AS PIDS
IF #Combined_Flag=1
BEGIN
ON PIDS.data_Period_dataination_Identifier = APEL.data_Period_dataination_Identifier
AND PIDS.data_Period_Identifier = APL.data_Period_Identifier
END
Basically
if BIT=0 join with function 1 else join with function 2
I tried putting an IF Clause .. but it does not seem to work. What is the proper way to do it?

Answer 1

Just add your static condition as part of the join condition and use a LEFT JOIN to ensure it works with the missing row. You can then use a case expression in your select to obtain the correct column e.g.
SELECT
CASE WHEN F1.id IS NOT NULL THEN F1.MyColumn ELSE F2.MyColumn END
FROM ...
LEFT JOIN markcommon.Function1() AS F1
ON #Combined_Cohort = 1
AND {The rest of the join conditions}
LEFT JOIN markcommon.Function2() AS F2
ON #Combined_Cohort = 0
AND {The rest of the join conditions}

Answer 2

I think you can just use a LEFT JOIN:
ON APEL.data_Period_dataination_Lookup_ID = CCEL.data_Period_dataination_Lookup_ID LEFT JOIN
markcommon.GetPredecessordatadataIds() PIDS
ON #Combined_Cohort = 1 AND
PIDS.data_Period_dataination_Identifier = APEL.data_Period_dataination_Identifier AND
PIDS.data_Period_Identifier = APL.data_Period_Identifier
. . .
WHERE #Combined_Cohort <> 1 OR PIDS.data_Period_dataination_Identifier IS NOT NULL

Answer 3

You can use the LEFT JOIN as follows:
APEL.data_Period_dataination_Lookup_ID = CCEL.data_Period_dataination_Lookup_ID
LEFT JOIN markcommon.GetPredecessordatadataIds() AS PIDS
ON (#Combined_Flag=1 AND PIDS.data_Period_dataination_Identifier = APEL.data_Period_dataination_Identifier
AND PIDS.data_Period_Identifier = APL.data_Period_Identifier
)
LEFT JOIN FUNCTION2() AS PIDS2
ON (#Combined_Flag=0 AND <<join condition for function 2>>
)

Answer 4

Please also check:
SELECT
IIF(F1.id IS NOT NULL,F1.MyColumn,F2.MyColumn)
FROM ...
LEFT JOIN markcommon.Function1() AS F1
ON #Combined_Cohort = 1
AND {The rest of the join conditions}
LEFT JOIN markcommon.Function2() AS F2
ON #Combined_Cohort != 1
AND {The rest of the join conditions}