Left outer join/left join
A left outer join returns all the rows from the left table that are specified in the left outer join clause, not just the rows in which the columns match.
A left outer join selects all the rows from the table on the left
(cows_one in the sample), and displays the matching rows from the
table on the right (cows_two). It displays an empty row for the table
on the right if the table has no matching row for the table on the
left.
SELECT * FROM cows_one LEFT OUTER JOIN cows_two ON cows_one.cnumber =
cows_two.cnumber;
cnumber | cbreed |cnumber | breeds
----------- +------------+ ---------+---------
1 | Holstein | |
2 | Guernsey | 2 | Jersey
3 | Angus | 3 | Brown Swiss
(3 rows)The keyword outer is optional.