table_reference RIGHT [OUTER] JOIN table_reference [join_condition]
table_reference NATURAL [RIGHT [OUTER]] JOIN table_reference
Where 
table_reference 
is defined as:
table_name [[AS] alias] [[USE INDEX (key_list)] | [IGNORE INDEX (key_list)] | [FORCE 
IN 
and 
join_condition 
is defined as:
ON conditional_expr |
USING (column_list)
The 
STRAIGHT_JOIN 
keyword forces the MySQL optimizer to join tables in the 
specified order. In DB2 the join order is always determined by the optimizer. The 
optimizer choices can be limited by changing the default query optimization class 
using SET CURRENT QUERY OPTIMIZATION.
A 
NATURAL
 join, as its name implies, can be invoked when two or more tables
share exactly the same columns needed for a successful equijoin. It is 
semantically equivalent to DB2 INNER JOIN or LEFT OUTER JOIN with the 
respective join criteria specified in the ON clause.
According to the SQL ANSI standard when you need to join tables that share 
more than one column naturally, the JOIN ... USING syntax needs to be used. An 
equivalent join can be composed using the DB2 supported join syntax in the ON 
clause.
Everybody in application and database query development spends a lot of time 
trying to avoid them; however, cartesian products do happen from time to time, 
usually as the result of an equijoin condition that has been missed in a query 
using DB2 syntax. However, one of the advantages of the 
CROSS JOIN
 syntax is 
that a specific keyword is required to create a Cartesian product. Therefore, 
when the 
CROSS JOIN
 syntax is used in your migration project, just code a regular 
join in DB2 with a no join condition in the WHERE clause.
7.1.3  UNION Syntax
In MySQL Version 4.0.0 the newly implemented UNION feature (shown in 
Example 7 5) is very similar to the DB2 syntax, and therefore does not require 
additional discussion.
Example 7 5   UNION syntax in MySQL and DB2
SELECT ...
UNION [ALL | DISTINCT]
SELECT ...
[UNION [ALL | DISTINCT]
SELECT ...]
158 
MySQL to DB2 UDB Conversion Guide






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