Joins Versus NOT IN Clause

Kevin Hill explains a potential performance difference between using NOT IN and using a left join:

Basic stuff, right?   Both will return 951 records (books) that I do not own.  And, very quickly…because the tables are tiny.   Sub-1 second is fast.

The issue here is HOW the rows are compared.

English version now, techy stuff later:

In the first query, this is equivalent to you standing at the bookstore and calling home to have someone check to see if the book in your hand is already in your collection.  EVERY time.  One by one.

In the second, you got really smart and brought a list with you, which you are comparing to the books on the shelf at the store.   You’ve got both “lists” in one place, so it is far more efficient.

Even in the case with a few hundred records, you can see why there’d be a performance difference.

Related Posts

Dealing With Large JSON Values

Bert Wagner investigates an issue he found where his long JSON strings were becoming NULL in SQL Server: After a little bit more research, I discovered that the return type for JSON_VALUE is limited to 4000 characters.   Since JSON_VALUE is in lax mode by default, if the output has more than 4000 characters, it fails silently. […]

Read More

Deleting Top Records With An Order By Clause

Kenneth Fisher shows that deleting the top N records with an ORDER BY clause is not straightforward: Did you know you can’t do this? DELETE TOP (10) FROM SalesOrderDetail ORDER BY SalesOrderID DESC; Msg 156, Level 15, State 1, Line 8 Incorrect syntax near the keyword ‘ORDER’. I didn’t. Until I tried it anyway. Turns […]

Read More

Categories

August 2016
MTWTFSS
« Jul Sep »
1234567
891011121314
15161718192021
22232425262728
293031