No CROSS JOIN in MS Access

For the upcoming jOOQ 3.3, we’re now integrating support for the MS Access database through the JDBC-ODBC bridge, which is included in the JDK up until Java SE 7. Note that it will be removed in Java 8! Alternative access to access databases (pun intended) can be obtained through a hack involving ucanaccess, which is basically combining the HSQLDB parser with Jackcess, a low-level I/O library for MS Access database files.

MS Access is still an immensely popular relational database, ranking in the top 10 at db-engines.com’s DBMS ranking. Yet it has quirks. Many of them. One is the fact that there is no formal CROSS JOIN operation. Remember, most databases support explicit CROSS JOINing as such:

SELECT     p1.name player1, 
           p2.name player2
FROM       player p1 
CROSS JOIN player p2

The above query will generate all the pairings between two players.
The same can be written in pre-ANSI SQL-92 with comma-separated table lists:

SELECT     p1.name player1, 
           p2.name player2
FROM       player p1, 
           player p2

The first syntax, however, is more powerful and more expressive as it can be used in nested JOIN expressions and it shows your intent more clearly. A nice example was given here in a previous blog post.

How to work around a missing CROSS JOIN

Usually, missing support for CROSS JOIN can be emulated trivially using an INNER JOIN with a TRUE predicate as such:

SELECT     p1.name player1, 
           p2.name player2
FROM       player p1 
JOIN       player p2
ON         1 = 1

This is what jOOQ does for the Sybase Adaptive Server Enterprise database. But this doesn’t work for MS Access, because the JOIN operation there explicitly requires column references from either table on either side. The documentation reads:

Syntax:

FROM table1 INNER JOIN table2 
ON table1.field1 compopr table2.field2

This is quite a bummer from many points of view, not only for CROSS JOIN emulation. Given that any ANSI-92 JOIN syntax can be transformed into an ANSI-86 join expression (table list in the FROM clause and all predicates in the WHERE clause), it is also a bit surprising.

A simple workaround that seems to work for some use-cases is to take any numeric column from either table, and multiply it by zero:

SELECT     p1.name player1, 
           p2.name player2
FROM       player p1 
JOIN       player p2
ON         p1.id * 0 = p2.id * 0

But if the database itself is already this quirky, I suspect that it might not be able to optimise the above SQL.

In short…

MS Access does not support CROSS JOIN. For the time being, try to work around it using comma-separated table lists, while we work out more sophisticated SQL transformation in jOOQ.