Question

Whats the difference between using a LEFT OUTER JOIN, rather than a sub-query that starts with a WHERE NOT EXISTS (...)?

Answer 1

No they are not the same thing, as they will not return the same rowset in the most simplistic use case.

The LEFT OUTER JOIN will return all rows from the left table, both where rows exist in the related table and where they does not. The WHERE NOT EXISTS() subquery will only return rows where the relationship is not met.

However, if you did a LEFT OUTER JOIN and looked for IS NULL on the foreign key column in the WHERE clause, you can make equivalent behavior to the WHERE NOT EXISTS.

For example this:

SELECT 
  t_main.*
FROM 
   t_main
   LEFT OUTER JOIN t_related ON t_main.id = t_related.id
/* IS NULL in the WHERE clause */
WHERE t_related.id IS NULL

Is equivalent to this:

SELECT
  t_main.*
FROM t_main 
WHERE 
  NOT EXISTS (
    SELECT t_related.id 
    FROM t_related 
    WHERE t_main.id = t_related.id
  )

But this one is not equivalent:

It will return rows from t_main both having and not having related rows in t_related.

SELECT 
  t_main.*
FROM
  t_main
  LEFT OUTER JOIN t_related ON t_main.id = t_related.id
/* WHERE clause does not exclude NULL foreign keys */

Note This does not speak to how the queries are compiled and executed, which differs as well — this only addresses a comparison of the rowsets they return.