Question

I have to write a LEFT JOIN query of find the records that do exist in one table and exists in other table (standard use for left join). There is only one table in this one for the join.

Table name: products

Join condition column: prdname

Two where clause conditions: subdivision='abc', subdivision='xyz'

Objecttive

With these two where clauses I will get two results containing prdname of two sets, I have to find prdname which exits in one but does not in another.

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When I wrote the standard LEFT JOIN (case 2 below), it does not give any results that were incorrect so I wrote a subquery to check the results. It was 8 records so I decided to modify the query to case1 down here, and it worked.

My doubt is that both of them are pretty much the same so why second is incorrect?

Can any one help me?

Also, abc and xyz are random values so bother about them much.

Case in which It is working:

SELECT a.prdname
FROM   products a
       LEFT OUTER JOIN (SELECT prdname
                        FROM   products
                        WHERE  subdivision = 'xyz') b
                    ON a.prdname = b.prdname
WHERE  a.subdivision = 'abc'
       AND b.prdname IS NULL 

Case in which it is not working

SELECT DISTINCT( a.prdname )
FROM   products a
       LEFT OUTER JOIN products b
                    ON a.prdname = b.prdname
WHERE  a.subdivision = 'abc'
       AND b.subdivision = 'xyz'
       AND b.prdname IS NULL 

Subquery Query to test

SELECT DISTINCT( prdname )
FROM   products
WHERE  subdivision = 'abc'
       AND prdname NOT IN (SELECT DISTINCT prdname
                           FROM   products
                           WHERE  subdivision = 'xyz') 

Answer 1

By adding a WHERE condition on columns of the “outer” table you are essentially turning the outer join into an inner join because those rows that do not satisfy the join criteria will have a NULL value in those columns. And the comparison with NULL removes those rows from the result.

You need to move that condition into the JOIN clause:

SELECT a.prdname
FROM   products a
  LEFT JOIN products b
         ON a.prdname = b.prdname
        AND b.subdivision = 'xyz'
WHERE  a.subdivision = 'abc'
   AND b.prdname IS NULL;

To make things a bit clearer, here is a simplified example.

Assuming the following tables:

Person

id | firstname | lastname  
---+-----------+-----------
1  | Arthur    | Dent      
4  | Mary      | Moviestar 
2  | Zaphod    | Beeblebrox
3  | Tricia    | McMillian 

Vehicle

id | person_id | vehicle_name
---+-----------+-------------
1  | 1         | Arthur's car
2  | 2         | Zaphod's car

Now fetch all people and their vehicles:

select p.id,
       p.firstname, 
       v.vehicle_name
from person p 
   left outer join vehicle v on p.id = v.person_id;

This returns the following:

id | firstname | vehicle_name
---+-----------+-------------
1  | Arthur    | Arthur's car
2  | Zaphod    | Zaphod's car
3  | Tricia    | (null)      
4  | Mary      | (null)      

Now think about what happens when you add

and v.vehicle_name = 'Arthur''s car' 

to the query. The two rows where vehicle_car is null won’t satisfy that condition and thus are removed from the result.

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Now to your real question:

To find products with subdivision ‘abc’ which do not exist in subdivision ‘xyz’ you can use the except operator:

select prdname
from products
where subdivision = 'abc'
except
select prdname
from products
where subdivision = 'xyz';

(For Oracle you must replace except with minus)