Can I get better performance using a JOIN or using EXISTS?

Question

I have two tables Institutions and Results and I want to see if there are any results for institutions that way I can exclude the ones that don\'t have results.

Can

Answer 1

A LEFT OUTER JOIN will tend to perform better than a NOT EXISTS**, but in your case you want to do EXISTS and using a simple INNER JOIN doesn't exactly replicate the EXISTS behavior. If you have multiple Results for an Institution, doing the INNER JOIN will return multiple rows for that institution. You could get around that by using DISTINCT, but then the EXISTS will probably be better for performance anyway.

** For those not familiar with this method:

SELECT
     MyTable.MyTableID
FROM
     dbo.MyTable T1
LEFT OUTER JOIN dbo.MyOtherTable T2 ON
     T2.MyTableID = T1.MyTableID
WHERE
     T2.MyOtherTableID IS NULL

is equivalent to

SELECT
     MyTable.MyTableID
FROM
     dbo.MyTable T1
WHERE NOT EXISTS (SELECT * FROM MyOtherTable T2 WHERE T2.MyTableID = T1.MyTableID)

assuming that MyOtherTableID is a NOT NULL column. The first method generally performs faster than the NOT EXISTS method though.