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JOIN Syntax
MySQL supports the following JOIN syntaxes for use in
SELECT statements:
table_reference, table_reference
table_reference [INNER | CROSS] JOIN table_reference [join_condition]
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [LEFT [OUTER]] JOIN table_reference
{ OJ table_reference LEFT OUTER JOIN table_reference ON conditional_expr }
table_reference RIGHT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference
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Where table_reference is defined as:
table_name [[AS] alias] [[USE INDEX (key_list)] | [IGNORE INDEX (key_list)] | [FORCE INDEX (key_list)]] |
and join_condition is defined as:
ON conditional_expr | USING (column_list) |
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:
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;
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ON conditional is any conditional of the form that may be used in
a WHERE clause.
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;
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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. See section 7.2.8 How MySQL Optimizes LEFT JOIN and RIGHT JOIN.
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 |
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.
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.
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:
mysql> SELECT * FROM table1,table2 WHERE table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> SELECT * FROM table1 LEFT JOIN table2 USING (id);
mysql> SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
-> LEFT JOIN table3 ON table2.id=table3.id;
mysql> SELECT * FROM table1 USE INDEX (key1,key2)
-> WHERE key1=1 AND key2=2 AND key3=3;
mysql> SELECT * FROM table1 IGNORE INDEX (key3)
-> WHERE key1=1 AND key2=2 AND key3=3;
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See section LEFT JOIN optimisation.
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