Semi-join is an efficient way to unnest an IN/EXISTS subquery. For example,
where A.id IN
where B.date> '2009-10-01');
returns from A whose ID is in the set of IDs found in B, whose date is greater than a certain date. This query can be unnested using a INNER join or LEFT OUTER JOIN, but we need to deduplicate the IDs returned by the subquery on table B. The semantics of LEFT SEMI JOIN is that as long as there is ANY row in the right-hand table that matches the join key, the left-hand table row will be emitted as a result w/o necessarily looking further in the right-hand table for further matches. This is exactly the semantics of the IN subquery.