SQL Server - only join if condition is met - sql

Question

I have three tables (at least, something similar) with the following relationships:
Item table:
ID | Val
---------+---------
1 | 12
2 | 5
3 | 22
Group table:
ID | Parent | Range
---------+---------+---------
1 | NULL | [10-30]
2 | 1 | [20-25]
3 | NULL | [0-15]
GroupToItem table:
GroupID | ItemID
---------+---------
1 | 1
1 | 3
And now I want to add rows to the GroupToItem table for Groups 2 and 3, using the same query (since some other conditions not shown here are more complicated). I want to restrict the items through which I search if the new group has a parent, but to look through all items if there is not.
At the moment I am using an IF/ELSE on two statements that are almost exactly the same, but for the addition of another JOIN row when a parent exists. Is it possible to do a join to reduce the number of items to look at, only if a restriction is possible?
My two queries as they stand are given below:
DECLARE #GroupID INT = 2;...
INSERT INTO GroupToItem(GroupID, ItemID)
SELECT g.ID,
i.ID,
FROM Group g
JOIN Item i ON i.Val IN g.Range
JOIN GroupToItem gti ON g.Parent = gti.GroupID AND i.ID = gti.ItemID
WHERE g.ID = #GroupID
-
DECLARE #GroupID INT = 3;...
INSERT INTO GroupToItem(GroupID, ItemID)
SELECT g.ID,
i.ID,
FROM Group g
JOIN Item i ON i.Val IN g.Range
WHERE g.ID = #GroupID
So essentially I only want to do the second JOIN if the given group has a parent. Is this possible in a single query? It is important that the number of items that are compared against the range is as small as possible, since for me this is an intensive operation.
EDIT: This seems to have solved it in this test setup, similar to what was suggested by Denis Valeev. I'll accept if I can get it to work with my live data. I've been having some weird issues - potentially more questions coming up.
SELECT g.Id,
i.Id
FROM Group g
JOIN Item i ON (i.Val > g.Start AND i.Val < g.End)
WHERE g.Id = 2
AND (
(g.ParentId IS NULL)
OR
(EXISTS(SELECT 1 FROM GroupToItem gti WHERE g.ParentId = gti.GroupId AND i.Id = gti.ItemId))
)
SQL Fiddle

Answer 1

Try this:
INSERT INTO GroupToItem(GroupID, ItemID)
SELECT g.ID,
i.ID,
FROM Group g
JOIN Item i ON i.Val IN g.Range
WHERE g.ID = #GroupID
and (g.ID in (3) or exists (select top 1 1 from GroupToItem gti where g.Parent = gti.GroupID AND i.ID = gti.ItemID))

Answer 2

If a Range column is a varchar datatype, you can try something like this:
INSERT INTO GROUPTOITEM (GROUPID, ITEMID)
SELECT A.ID, B.ID
FROM GROUP AS A
LEFT JOIN ITEM AS B
ON B.VAL BETWEEN CAST(SUBSTRING(SUBSTRING(A.RANGE,1,CHARINDEX('-',A.RANGE,1)-1),2,10) AS INT)
AND CAST(REPLACE(SUBSTRING(A.RANGE,CHARINDEX('-',A.RANGE,1)+1,10),']','') AS INT)