SQL Sub-query or INNER-JOIN?

Question

I\'ve the two following queries:

declare @UserId as int
set @UserId = 1

-- Query #1: Sub-query
SELECT
    u.[Id] ,
    u.[Name] ,
    u.[OrgId] AS Organizat         


        

Answer 1

The relative cost of an execution plan isn't always a reliable indicator of performance.

I assume from your sql that only 1 row should be returned. Providing that the UserId is a unique key on User, then the performance of your 2 approaches will be similar on most relational databases.

Things to bear in mind would be:

• if UserPreferences or SearchCriteria return more than 1 row, the first approach will raise an sql error, the second approach will return more than 1 row.
• the apparent extra lookup in the first approach (UserPreferences selected twice) has no real effect because for the second lookup the record will already be in a buffer
• if for some reason the User table is tablespace scanned, the first approach will be much faster