Are left outer joins and left joins the same? [duplicate]

Answer 1

In MySQL syntax, LEFT OUTER JOIN and LEFT JOIN are identical: (from http://dev.mysql.com/doc/refman/5.0/en/join.html)

| table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
| table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

Note that the OUTER keyword is optional for LEFT/RIGHT JOIN.

LEFT and RIGHT joins are both outer joins. I believe there are some flavors of SQL that may require the OUTER keyword, but MySQL does not. That is to say that at times LEFT JOIN may not be valid.

Answer 2

There are no difference between both. Refer visual represenation of joins

Answer 3

The first link they quoted gives you:

INNER JOIN: returns rows when there is a match in both tables.

LEFT JOIN / LEFT OUTER JOIN: returns all rows from the left table, even if there are no matches in the right table.

RIGHT JOIN / RIGHT OUTER JOIN: returns all rows from the right table, even if there are no matches in the left table.

FULL JOIN / FULL OUTER JOIN / OUTER JOIN: returns rows when there is a match in one of the tables.

SELF JOIN: is used to join a table to itself, as if the table were two tables, temporarily renaming at least one table in the SQL statement.

CARTESIAN JOIN: returns the cartesian product of the sets of records from the two or more joined tables.

The self join is actually not a special join. It just reflects the fact that you can join a table with itself. Doing so you must alias it in order to address the fact that it appears more than once in the same statement.

The cartesian join can be considered as an inner join without a restricting condition. Or you may view an inner join as a cartesian join with an added restriction (the join condition).

Answer 4

If you look at the manual page for JOIN you will see the following lines:

join_table:

   table_reference [INNER | CROSS] JOIN table_factor [join_condition]

   | table_reference STRAIGHT_JOIN table_factor

   | table_reference STRAIGHT_JOIN table_factor ON conditional_expr

| table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition

   | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

The bold line clearly shows that the keyword OUTER is optional.

Answer 5

I am working in SQL Server and as per my usage and experience there is absolutely no difference between LEFT JOIN and LEFT OUTER JOIN. The same is true for RIGHT JOIN and RIGHT OUTER JOIN. When you use LEFT JOIN keyword in SQL Server, it means LEFT OUTER JOIN only. So as per my opinion its the generic rule which is same for all database engines.

see here and here