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SQL Anywhere 12.0.0 (Français) » SQL Anywhere Server - SQL Usage » Querying and modifying data » Joins: Retrieving data from several tables » Specialized joins

 

Duplicate correlation names in joins (star joins)

The reason for using duplicate table names is to create a star join. In a star join, one table or view is joined to several others.

To create a star join, you use the same table name, view name, or correlation name more than once in the FROM clause. This is an extension to the ANSI/ISO SQL standard. The ability to use duplicate names does not add any additional functionality, but it makes it easier to formulate certain queries.

The duplicate names must be in different joins for the syntax to make sense. When a table name or view name is used twice in the same join, the second instance is ignored. For example, FROM A,A and FROM A CROSS JOIN A are both interpreted as FROM A.

The following example, in which A, B and C are tables, is valid in SQL Anywhere. In this example, the same instance of table A is joined both to B and C. Note that a comma is required to separate the joins in a star join. The use of a comma in star joins is specific to the syntax of star joins.

SELECT *
FROM A LEFT OUTER JOIN B ON A.x = B.x,
     A LEFT OUTER JOIN C ON A.y = C.y;

The next example is equivalent.

SELECT *
FROM A LEFT OUTER JOIN B ON A.x = B.x,
     C RIGHT OUTER JOIN A ON A.y = C.y;

Both of these are equivalent to the following standard ANSI/ISO syntax. (The parentheses are optional.)

SELECT *
FROM (A LEFT OUTER JOIN B ON A.x = B.x)
LEFT OUTER JOIN C ON A.y = C.y;

In the next example, table A is joined to three tables: B, C and D.

SELECT *
FROM A JOIN B ON A.x = B.x,
     A JOIN C ON A.y = C.y,
     A JOIN D ON A.w = D.w;

This is equivalent to the following standard ANSI/ISO syntax. (The parentheses are optional.)

SELECT *
FROM ((A JOIN B ON A.x = B.x)
JOIN C ON A.y = C.y)
JOIN D ON A.w = D.w;

With complex joins, it can help to draw a diagram. The previous example can be described by the following diagram, which illustrates that tables B, C and D are joined via table A.

Table A is in the center, with B, C, and D each connecting to it.
Note

You can use duplicate table names only if the extended_join_syntax option is On (the default).

For more information, see extended_join_syntax option.

 Example 1
 Example 2