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Re: Is not equal to query...

From: Merlin Moncure <mmoncure(at)gmail(dot)com>
To: James David Smith <james(dot)david(dot)smith(at)gmail(dot)com>
Cc: pgsql novice <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Is not equal to query...
Date: 2011-06-09 16:27:50
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Lists: pgsql-novice
On Thu, Jun 9, 2011 at 6:17 AM, James David Smith
<james(dot)david(dot)smith(at)gmail(dot)com> wrote:
> Hi Merlin,
> Thank you very much for the full and brilliant reply. The last query
> you wrote does exactly what I want it too. I wonder whether it's not
> too much trouble whether you could explain to me a couple of things
> though...?
> 1) I understand what you are saying my query does. Like an outer join
> I think? But don't get why. Using '!=' is the opposite of '=' is it
> not?

It is the opposite -- just not in the way you are thinking.  In SQL,
joins between tables means 'give me every combination of data from
table A combined with table B given a condition'.  If A and B each
have 100 records with identifiers 1-100, the not equal join would give
you a join result of 99 records for A=1 (with B 2-100), 99 records for
A=2 (with B 1, 3-100) etc. for a total of 9900 records.

Your problem is that you are still associating tables A and B in your
head in a way that is not expressed in the join.  You have to imagine
both tables as pools of unassociated records with no ordering except
for what you give in the query.

> 2) I understand the 'where not exists' query you suggest, and have
> used that, but I don't see why you use '1' in it. What does the 1 do?

where not exists means 'return this record if this query does not
return at least 1 record' -- since we don't care what is in the record
that is actually returned, I just use 1 as a shorthand because in SQL
you can't write queries that return 0 fields -- this is a pretty
common convention.  I instead of 1, I could have used *, -999, or
anything -- it doesn't matter.

Aside: exists/not exists in relational parlance are called 'semi
joins'.  They can often be faster than regular joins because the
server can bail early without having to express the full join.


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