Re: Left Outer Join Syntax

Tom,
Thanks for that.
I'll be testing the converted system thoroughly, so should pick up all
the anomalies that I've introduced!
I can now finish off some of the more obscure joins in the code before I
start the data import and then testing.

On Sat, 2004-08-14 at 16:44, Tom Lane wrote:

Steve Tucknott <steve(at)retsol(dot)co(dot)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

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Regards,

Steve Tucknott

ReTSol Ltd

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