There are three types of join
There are 3 kinds of join INNER, LEFT OUTER, RIGHT OUTER and each
requires an ON clause to let the RDBMS know what fields to use joining
the tables. For each join there are two table the left table and the
right table. The syntax being the following


{left table} {INNER | LEFT | RIGHT} JOIN {right table} ON {join
criteria}
An INNER JOIN returns only those rows from left table where they have a
matching row in right table based on the join criteria.
A LEFT JOIN returns ALL rows from the left table even if no matching
rows where found in the right table. Any values selected out of the
right table will be null for those rows where no matching row is found
in the right table.
A RIGHT JOIN works exactly the same as a left join but reversing the
direction. So it would return all rows in the right table regardless of
matching rows in the left table. It is recommended that you no use
right joins as your query can always be rewritten to use left joins
which tend to be more portable and easier to read.
With all of the joins if there are multiple rows in one table that
match one row in the other table will result in that row getting
returned many time.
For example:
Table A
tid, name
1, 'Linux'
2, 'Debian'
Table B
fid, tid, message
1, 1, 'Very Cool'
2, 1, 'What an example'
Query 1:
SELECT a.name, b.message FROM a INNER JOIN b ON a.tid = b.tid
Result 1:
Linux, Very Cool
Linux, What an example
Query 2:
SELECT a.name, b.message FROM a LEFT JOIN b ON a.tid = b.tid
Result 2:
Linux, Very Cool
Linux, What an example
Debian,