Outer joins

In Informix^® SQL, outer tables can be defined in the FROM clause with the OUTER keyword:

SELECT ... FROM a, OUTER(b)
 WHERE a.key = b.akey

SELECT ... FROM a, OUTER(b,OUTER(c))
 WHERE a.key = b.akey    AND b.key1 = c.bkey1
   AND b.key2 = c.bkey2

MySQL 3.23 supports the ANSI outer join syntax:

SELECT ... FROM cust LEFT OUTER JOIN order
                      ON cust.key = order.custno
SELECT ...
  FROM cust LEFT OUTER JOIN order
                  LEFT OUTER JOIN item
                  ON order.key = item.ordno
             ON cust.key = order.custno
 WHERE order.cdate > current date

See the MySQL reference for a complete description of the syntax.

Solution

For better SQL portability, you should use the ANSI outer join syntax instead of the old Informix OUTER syntax.

The MySQL interface can convert most Informix OUTER specifications to ANSI outer joins.

Prerequisites:

In the FROM clause, the main table must be the first item and the outer tables must figure from left to right in the order of outer levels.
Example which does not work: "FROM OUTER(tab2), tab1".
The outer join in the WHERE part must use the table name as prefix.
Example: "WHERE tab1.col1 = tab2.col2".

Restrictions:

Additional conditions on outer table columns cannot be detected and therefore are not supported:
Example: "... FROM tab1, OUTER(tab2) WHERE tab1.col1 = tab2.col2 AND tab2.colx > 10".
Statements composed by 2 or more SELECT instructions using OUTERs are not supported.
Example: "SELECT ... UNION SELECT" or "SELECT ... WHERE col IN (SELECT...)"

Note:

Table aliases are detected in OUTER expressions.
OUTER example with table alias: "OUTER( tab1 alias1)".
In the outer join, <outer table>.<col> can be placed on both right or left side of the equal sign.
OUTER join example with table on the left: "WHERE outertab.col1 = maintab.col2 ".
Table names detection is not case-sensitive.
Example: "SELECT ... FROM tab1, TAB2 WHERE tab1.col1 = tab2.col2".
Temporary tables
are supported in OUTER specifications.