cification] [[AS] alias] [index_hint_list]
 | table_subquery [query_system_time_period_specification] [AS] alias
 | ( table_references )
 | { ON table_reference LEFT OUTER JOIN table_reference
    ON conditional_expr }

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

join_condition:
  ON conditional_expr
 | USING (column_list)

query_system_time_period_specification:
  FOR SYSTEM_TIME AS OF point_in_time
 | FOR SYSTEM_TIME BETWEEN point_in_time AND point_in_time
 | FOR SYSTEM_TIME FROM point_in_time TO point_in_time
 | FOR SYSTEM_TIME ALL

point_in_time:
  [TIMESTAMP] expression
 | TRANSACTION expression

index_hint_list:
  index_hint [, index_hint] ...

index_hint:
  USE {INDEX|KEY}
   [{FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
 | IGNORE {INDEX|KEY}
   [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
 | FORCE {INDEX|KEY}
   [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
  index_name [, index_name] ...

A table reference is also known as a join expression.

Each table can also be specified as db_name.tabl_name. This allows to write
queries which involve multiple databases. See Identifier Qualifiers for syntax
details.

The syntax of table_factor is extended in comparison with the SQL Standard.
The latter accepts only table_reference, not a list of them inside a pair of
parentheses.

This is a conservative extension if we consider each comma in a list of
table_reference items as equivalent to an inner join. For example:

SELECT * FROM t1 LEFT JOIN (t2, t3, t4)
        ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

is equivalent to:

SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4)
        ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

In MariaDB, CROSS JOIN is a syntactic equivalent to INNER JOIN (they can
replace each other). In standard SQL, they are not equivalent. INNER JOIN is
used with an ON clause, CROSS JOIN is used otherwise.

In general, parentheses can be ignored in join expressions containing only
inner join operations. MariaDB also supports nested joins (see
http://dev.mysql.com/doc/refman/5.1/en/nested-join-optimization.html).

See System-versioned tables for more information about FOR SYSTEM_TIME syntax.

Index hints can be specified to affect how the MariaDB optimizer makes use of
indexes. For more information, see How to force query plans.

Examples
--------

SELECT left_tbl.*
 FROM left_tbl LEFT JOIN right_tbl ON left_tbl.id = right_tbl.id
 WHERE right_tbl.id IS NULL;

URL: https://mariadb.com/kb/en/join-syntax/https://mariadb.com/kb/en/join-syntax/