Re: Left Outer Join Syntax

Steve Tucknott <steve@retsol.co.uk> writes:
> How do I include the join of table F to table D where F.colD = D.colF in
> the case where 1) F is a LEFT OUTER and 2) where F is plain (INNER?)
> join

I think you just want to parenthesize the join constructs:

    (a left join (f left join d on somecondition) on somecondition)
or
    (a left join (f join d on somecondition) on somecondition)

However you need to be clear in your mind about the semantic behavior
you want before you can pick a join order, and your question certainly
didn't give enough detail for anyone to offer advice.  In either one of
the above examples, D rows that don't have a join partner in F will
disappear before they get to the A join, resulting in different results
than you had before --- that is, some A rows that were joined to D rows
would now be extended with with nulls.  If any of those rows make it to
the final output then you will see a different and probably less useful
answer.

The short form of my point is that outer joins aren't associative and so
the order in which you do them matters a lot.  The reason JOIN is
syntactically like an operator is so that you can control that ordering
through parentheses.

            regards, tom lane