How do I replace this where clause with a join?
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16-10-2019 - |
Question
Typically when I see SQL that uses something like:
select * from employees where epmloyeeTypeId in (select id from type where name = 'emp')
I replace the where
with this:
select e.* from employees e
inner join type t on t.id=e.epmloyeeTypeId and t.name = 'emp'
Is it possible to do the same with the inverse in case its is a not in
(like below) instead of an in
clause?
INSERT into Subscriptions(ProjectId, RecordTypeCID, NTID, Active, Added, LastUpdate, UpdateBy)
SELECT @ProjectId, RecordTypeCID, @NTID, 1, GETDATE(), GETDATE(), @NTID
FROM @Check CHK
WHERE CHK.ActiveStatus=1
And Not Exists (SELECT SubscriptionId FROM Subscriptions
WHERE ProjectId=@ProjectId
and NTID=@NTID
and RecordTypeCID = CHK.RecordTypeCID
)
Additional considerations
Can I do this:
INSERT INTO Subscriptions(ProjectId, RecordTypeCID, NTID,Active, Added, LastUpdate, UpdateBy)
SELECT @ProjectId, RecordTypeCID, @NTID,1, GETDATE(), GETDATE(), @NTID
FROM @Check CHK
LEFT JOIN Subscriptions subs ON subs.RecordTypeCID = CHK.RecordTypeCID
AND NTID = @NTID
AND ProjectId = @ProjectId
AND CHK.ActiveStatus = 1
AND subs.SubscriptionId IS NULL
Solution
Yes. You can replace with a LEFT JOIN ... WHERE key IS NULL. Performs much faster.
INSERT INTO Subscriptions(
ProjectId, RecordTypeCID, NTID,
Active, Added, LastUpdate, UpdateBy)
SELECT @ProjectId, RecordTypeCID, @NTID,
1, GETDATE(), GETDATE(), @NTID
FROM @Check CHK
LEFT JOIN Subscriptions subs
ON subs.RecordTypeCID = CHK.RecordTypeCID
AND NTID = @NTID
AND ProjectId = @ProjectId
WHERE CHK.ActiveStatus = 1
AND subs.SubscriptionId IS NULL
OTHER TIPS
Your NOT EXISTS is more efficient in most cases.
LEFT JOIN internally matches all rows then filters to IS NULL. NOT EXISTS doesn't. This row multiplication also happens in all JOIN based code so you may need an extra aggregate (DISTINCT) to fix the output
NOT IN is generally wrong because NULLs cause no match.
You can also use EXCEPT which gives the same plan as NOT EXISTS.
SELECT @ProjectId, RecordTypeCID, @NTID,1, GETDATE(), GETDATE(), @NTID
FROM @Check CHK
EXCEPT
SELECT @ProjectId, RecordTypeCID, @NTID,1, GETDATE(), GETDATE(), @NTID
FROM Subscriptions
WHERE ProjectId=@ProjectId
and NTID=@NTID
FYI, as per the SQL-92 standard (page 191, Case 3a) the SELECT bit of the EXISTS is ignored.