Sunday, July 31, 2011

Outer joins and search predicates – II

In my last post we saw that if we place search predicate in WHERE clause instead of placing it along with ON clause, we may get expected output. Now let's understand what happens behind the scene.
Note : we'll use pubs database for the example. 

Suppose we want to list down all the book titles, then we might write following query.
SELECT T.title_id as ID, T.Title 
FROM titles T 

Now we are asked to list down sales quantity for any given store for all the titles. If the given title is not sold, the quantity should be zero. For that we might write following query.

SELECT T.title_id, T.title, IsNull(S.qty,0) Qty 
FROM titles T 
LEFT JOIN sales S on T.title_id = S.title_id 
WHERE S.stor_id = '6380' 
ORDER BY T.title 

If you run above query, you'll get only two rows which were sold instead of remaining titles with zero quantity. Now if you see the execution plan, you could see that LEFT JOIN is converted to INNER JOIN by the execution engine. Check out below snap.

Now if you need to get all the rows with quantity zero if title is not sold, just place your search predicate along with ON clause in LEFT JOIN and you'll get the expected output. So our expected query should be like below :

SELECT T.title_id, T.title, IsNull(S.qty,0) Qty 
FROM titles T 
LEFT JOIN sales S on T.title_id = S.title_id 
and S.stor_id = '6380' 
ORDER BY T.title 

Always be careful when using search conditions with OUTER JOIN and construct your queries

as per the output expected. Enjoy!! :)

Sunday, July 10, 2011

Outer joins and search predicates

We often use outer joins to construct queries and get expected output, but what if you get unexpected out and the query seem ok. Few days back I experienced the same thing and found the problem with the placement of predicate (search condition).

Let’s take an example :

We’ll use Orders and Customers table from Northwind database.

Let’s check out INNER JOIN and LEFT JOIN result without any condition first.


As you can see in the snap above both are returning same no of rows ( I’ve taken top(5) for the snap, as both the queries have more than 100 rows)

Now let’s check out INNER JOIN with predicate (search condition) in WHERE and ON clause.


Both the query returned the same result.

Now let’s check out LEFT JOIN with WHERE clause.


It has also returned the same no of rows, but if we use predicate (search condition) along with ON clause rather than WHERE clause, it will return more no of rows (unexpected output).


Explanation could be found out on SQL Server Books Online which says,

There can be predicates that involve only one of the joined tables in the ON clause. Such predicates also can be in the WHERE clause in the query. Although the placement of such predicates does not make a difference for INNER joins, they might cause a different result when OUTER joins are involved. This is because the predicates in the ON clause are applied to the table before the join, whereas the WHERE clause is semantically applied to the result of the join.

An outer join query can produce completely different results depending on how you write it, and where predicates are placed in that query. Take Care!