What is the difference between using a cross join and putting a comma between the two tables?

What is the difference between

select * from A, B

and

select * from A cross join B

? They seem to return the same results.

Is the second version preferred over the first? Is the first version completely syntactically wrong?

They return the same results because they are semantically identical. This:

select * 
  from A, B

...is (wince) ANSI-89 syntax. Without a WHERE clause to link the tables together, the result is a cartesian product. Which is exactly what alternative provides as well:

    select * 
      from A 
cross join B

...but the CROSS JOIN is ANSI-92 syntax.

About Performance

There's no performance difference between them.

Why Use ANSI-92?

The reason to use ANSI-92 syntax is for OUTER JOIN support (IE: LEFT, FULL, RIGHT)--ANSI-89 syntax doesn't have any, so many databases implemented their own (which doesn't port to any other databases). IE: Oracle's (+), SQL Server's =*

Stumbled upon this post from another SO question, but a big difference is the linkage cross join creates. For example using cross apply or another join after B on the first ('comma') variant, the cross apply or join would only refer to the table(s) after the dot. e.g, the following:

select * from A, B join C on C.SomeField = A.SomeField and C.SomeField = B.SomeField 

would create an error:

The multi-part identifier "A.SomeField" could not be bound.

because the join on C only scopes to B, whereas the same with cross join...

select * from A cross join B join C on C.SomeField = A.SomeField and C.SomeField = B.SomeField 

..is deemed ok. The same would apply if cross apply is used. For example placing a cross apply on a function after B, the function could only use fields of B, where the same query with cross join, could use fields from both A and B. Of course, this also means the reverse can be used as well. If you want to add a join solely for one of the tables, you can achieve that by going 'comma' on the tables.

They are the same and should (almost) never be used.

The first version was originally the only way to join two tables. But it has a number of problems so the JOIN keyword was added in the ANSI-92 standard. They give the same results but the second is more explicit and is to be preferred.