Question

Why is the order of tables important when combining an outer & an inner join ? the following fails with postgres:

SELECT grp.number AS number,             tags.value AS tag    FROM groups grp,      insrel archiverel   LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.dnumber    LEFT OUTER JOIN tags tags ON tags.number = ownrel.snumber    WHERE archiverel.snumber = 11128188 AND           archiverel.dnumber = grp.number  

with result:

ERROR:  invalid reference to FROM-clause entry for table 'grp' LINE 5: LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.d...  ^ HINT:  There is an entry for table 'grp', but it cannot be referenced from this part of the query. 

when the groups are reversed in the FROM it all works:

SELECT  grp.number AS number,              tags.value AS tag    FROM    insrel archiverel,         groups grp LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.dnumber    LEFT OUTER JOIN tags tags ON tags.number = ownrel.snumber    WHERE   archiverel.snumber = 11128188 AND             archiverel.dnumber = grp.number  

Answer 1

I believe that you can think of this as an operator precedence issue.

When you write this:

FROM groups grp,      insrel archiverel   LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.dnumber    LEFT OUTER JOIN tags tags ON tags.number = ownrel.snumber    

I think it is interpreted by the parser like this:

FROM groups grp, (   (      insrel archiverel        LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.dnumber      ) LEFT OUTER JOIN tags tags ON tags.number = ownrel.snumber ) 

If so, then in the innermost join ‘grp’ is unbound.

When you reverse the lines with ‘groups’ and ‘insrel’, the innermost join applies to ‘groups’ and ‘ownrel’, so it works.

Probably this would work as well:

    FROM groups grp          JOIN insrel archiverel  ON archiverel.dnumber = grp.number     LEFT OUTER JOIN ownrel ownrel ON grp.number = ownrel.dnumber        LEFT OUTER JOIN tags tags ON tags.number = ownrel.snumber  WHERE archiverel.snumber = 11128188