MySQL supports the following JOIN syntaxes for use in SELECT statements:

Where table_reference is defined as:

and join_condition is defined as:

You should generally not have any conditions in the ON part that are used to restrict which rows you want in the result set, but rather specify these conditions in the WHERE clause. There are exceptions to this rule.

Note that INNER JOIN syntax allows a join_condition only from MySQL 3.23.17 on. The same is true for JOIN and CROSS JOIN only as of MySQL 4.0.11.

The last LEFT OUTER JOIN syntax shown in the preceding list exists only for compatibility with ODBC:

• A table reference may be aliased using tbl_name AS alias_name or tbl_name alias_name : mysql> SELECT t1.name, t2.salary FROM employee AS t1, info AS t2 -> WHERE t1.name = t2.name;
• The ON conditional is any conditional of the form that may be used in a WHERE clause.
• If there is no matching record for the right table in the ON or USING part in a LEFT JOIN , a row with all columns set to NULL is used for the right table. You can use this fact to find records in a table that have no counterpart in another table: mysql> SELECT table1.* FROM table1 -> LEFT JOIN table2 ON table1.id=table2.id -> WHERE table2.id IS NULL; This example finds all rows in table1 with an id value that is not present in table2 (that is, all rows in table1 with no corresponding row in table2 ). This assumes that table2.id is declared NOT NULL .
• The USING (column_list) clause names a list of columns that must exist in both tables. The following two clauses are semantically identical: a LEFT JOIN b USING (c1,c2,c3) a LEFT JOIN b ON a.c1=b.c1 AND a.c2=b.c2 AND a.c3=b.c3
• The NATURAL [LEFT] JOIN of two tables is defined to be semantically equivalent to an INNER JOIN or a LEFT JOIN with a USING clause that names all columns that exist in both tables.
• INNER JOIN and , (comma) are semantically equivalent in the absence of a join condition: both will produce a Cartesian product between the specified tables (that is, each and every row in the first table will be joined onto all rows in the second table).
• RIGHT JOIN works analogously to LEFT JOIN . To keep code portable across databases, it's recommended to use LEFT JOIN instead of RIGHT JOIN .
• STRAIGHT_JOIN is identical to JOIN , except that the left table is always read before the right table. This can be used for those (few) cases in which the join optimizer puts the tables in the wrong order.
• As of MySQL Version 3.23.12, you can give hints about which index MySQL should use when retrieving information from a table. This is useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes. By specifying USE INDEX (key_list) , you can tell MySQL to use only one of the possible indexes to find rows in the table. The alternative syntax IGNORE INDEX (key_list) can be used to tell MySQL to not use some particular index. In MySQL 4.0.9 you can also use FORCE INDEX . This acts likes USE INDEX (key_list) but with the addition that a table scan is assumed to be VERY expensive. In other words a table scan will only be used if there is no way to use one of the given index to find rows in the table. USE/IGNORE KEY are synonyms for USE/IGNORE INDEX .

Note: USE/IGNORE/FORCE INDEX only affects which indexes are used when MySQL decides how to find rows in the table and how to do the join. It doesn't affect whether an index will be used when resolving an ORDER BY or GROUP BY .

Some examples:

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