LEFT JOIN and IS NULL effect

Question

At http://dev.mysql.com/doc/refman/5.5/en/left-join-optimization.html one can read:

If you use LEFT JOIN to find rows that do not exist in some table and you have the following test: col_name IS NULL in the WHERE part, where col_name is a column that is declared as NOT NULL, MySQL stops searching for more rows (for a particular key combination) after it has found one row that matches the LEFT JOIN condition.

Can any body explain me what that mean in a human readable language? or in a newbie readable language ;)

Answer 1

Only rows that has no match on the right table will only be returned. This can be illustrated as:

As you can see, there are two tables being joined. The shaded part are the records which do not exists on Table B.