Alternative to using subqueries in SQL statements? - sql

Question

I have two tables:
TableA: (a temporary table)
ItemId (int)
TableB:
ItemId (int), ParentID (int)
I want to retrieve all items in Table A where the ParentID of any of the items in Table A doesn't exist as an ItemID. (i.e. I want to get the root of the items in TableA)
This query does what I want:
SELECT a.ItemID
FROM TableA a
INNER JOIN TableB b ON a.ItemId = b.ItemID
WHERE b.ParentID NOT IN ( SELECT * from TableA )
as does this one:
SELECT b.ItemID
FROM TableB b
WHERE b.ItemID IN ( SELECT * FROM TableA)
AND b.ParentID NOT IN ( SELECT * FROM TableA )
I am not satisfied with either of the queries, particularly because of the use of NOT IN/IN. Is there a way to do this without them? Perhaps a cleaner way that doesn't require subqueries?
Sample Data:
Table A
-------
2
3
5
6
Table B
--------
1 | NULL
2 | 1
3 | 1
4 | 3
5 | 3
6 | 3
Desired Result:
2
3
Thanks

Answer 1

Without subqueries:
SELECT ItemID
FROM TableA
INTERSECT
SELECT b.ItemID
FROM TableB AS b
LEFT OUTER JOIN TableA AS a
ON b.ParentID = a.ItemID
WHERE a.ItemID IS NULL;
...but is your fear of subqueries rational? :) I'd find this equivalent query easier to read and understand:
SELECT ItemID
FROM TableA
INTERSECT
SELECT ItemID
FROM TableB
WHERE NOT EXISTS (
SELECT *
FROM TableA AS a
WHERE a.ItemID = TableB.ParentID
);

Answer 2

Take a look at Select all rows from one table that don't exist in another table to see 5 different ways to do this kind of query by using
NOT IN
NOT EXISTS
LEFT and RIGHT JOIN
OUTER APPLY (2005+)
EXCEPT (2005+)
Here is a script that you can run
CREATE TABLE #TableA( ItemId int)
INSERT #TableA values(1)
INSERT #TableA values(2)
INSERT #TableA values(3)
INSERT #TableA values(4)
INSERT #TableA values(5)
INSERT #TableA values(6)
CREATE TABLE #TableB( ItemId int, ParentID int)
INSERT #TableB values(1,1)
INSERT #TableB values(2,2)
INSERT #TableB values(4,3)
INSERT #TableB values(5,4)
this will do it for parent
SELECT a.ItemID
FROM #TableA a
LEFT JOIN #TableB b ON a.ItemId = b.ParentID
WHERE b.ItemID IS NULL
SELECT a.ItemID
FROM #TableA a
WHERE NOT EXISTS (SELECT 1 FROM #TableB b WHERE a.ItemId = b.ParentID)
Output
ItemID
5
6

Answer 3

You can use outer joins. Something like this:
SELECT a.ItemID
FROM TableA a
INNER JOIN TableB b ON a.ItemId = b.ItemID
LEFT JOIN TableB parentB on a.ItemID = parentB.ParentID
WHERE parentB.ParentID IS NULL

Answer 4

Your Tables A and B seem to store a tree structure. I'd interpret table A as "Nodes" (storing elements of the tree) and Table B as "Edges" (linking a node to it's parent). The inner join variant is very elegant since it covers all cases of "no edge to parent", "edge to PrantID null" and "edge to non existant parent" at once.
cheers