Question

I was wondering if someone could help me understand what this code is doing. I have inhereted a SQL script and I’m trying to update the code so it’s easier to read. I’ve looked at the below code and it seems like there’s an easier way to do this, but I can’t wrap my head around what this code is actually doing. Could anyone help me just describe this code? (Maybe clue me into how to do this with a subquery or EXISTS or an easier way to read this?

SELECT DISTINCT
    s.id
    ,f.FLAG1
    ,f.FLAG1
    ,f.FLAG1
INTO #DLK_TEMP
from Inner_Source D
    LEFT OUTER JOIN Outer_Source_1 S on D.au = S.AU 
        AND D.wcv_entity_key = S.wcv_entity_key 
    LEFT OUTER JOIN Outer_Source_2 F on S.id = F.id
WHERE S.id IS NOT NULL 

Answer 1

It seems strange that it’s performing an LEFT OUTER JOIN on S and then testing for those rows where the join succeeds. It should be an INNER JOIN instead. And if S.ID is not nullable then you can also remove the WHERE clause.

SELECT DISTINCT
    s.id
    ,f.FLAG1
    ,f.FLAG1
    ,f.FLAG1
INTO #DLK_TEMP
FROM Inner_Source AS D
JOIN Outer_Source_1 AS S
    ON D.au = S.AU 
    AND D.wcv_entity_key = S.wcv_entity_key 
LEFT OUTER JOIN Outer_Source_2 AS F
ON S.id = F.id

If you like, you can change the first join to an EXISTS clause to make it clear that the join is only acting as a filter:

SELECT DISTINCT
    s.id
    ,f.FLAG1
    ,f.FLAG1
    ,f.FLAG1
INTO #DLK_TEMP
FROM Outer_Source_1 AS S
LEFT OUTER JOIN Outer_Source_2 AS F ON S.id = F.id
WHERE EXISTS
(
    SELECT *
    FROM Inner_Source AS D
    WHERE D.au = S.AU 
    AND D.wcv_entity_key = S.wcv_entity_key
)