Re: conditional FROM

On 2011-12-10, Richard Klingler <richard(at)klingler(dot)net> wrote:
> Good day...
>
> I'm trying to build a query for PGSQL 9.1 where a table has two
> references with only one being used depending of the type of entry..
>
> For example, the table has following simplified structure:
>
> portid primary key
> port2node index to table node
> port2card index to table card
>
> So how can I do a conditional FROM clause in the query depending on the
> column port2node and port2card?
> If port2card is Null or 0 I don't want it in the FROM clause as the
> query will return unneccessary duplicate
> row...the same goes for port2node being Null or 0...

use left outer join.

SELECT * FROM
port
LEFT OUTER JOIN node ON node.nodeid=port.port2node
LEFT OUTER JOIN card ON card.cardid=port.port2card

or something like that.

You may find coalesce() useful to combine columns where node and card
both carry equivalent information.

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