18. Video: JOINs and Filtering

A simple rule to remember this is that, when the database executes this query, it executes the join and everything in the ON clause first. Think of this as building the new result set. That result set is then filtered using the WHERE clause.

The fact that this example is a left join is important. Because inner joins only return the rows for which the two tables match, moving this filter to the ON clause of an inner join will produce the same result as keeping it in the WHERE clause.