Re: joining from multiple tables

On Wed, 15 Jan 2003, Joseph Shraibman wrote:

> select distinct n.thekey, n.val, t.txt FROM num_tab n LEFT JOIN txt_tab t ON n.thekey =
> t.thekey
> WHERE n.thekey < 5 AND t.class = class_tab.tkey AND n.class = class_tab.class;
> produces:

Note that the above uses the non-standard postgres behavior of adding from
clauses, it's not technically valid SQL.

> thekey | val | txt
> --------+-----+------
> 2 | 2 | two
> 4 | 4 | four
> ... which is not what we want, because 1,3, and 5 aren't included, but:
>
> select distinct n.thekey, n.val, t.txt FROM num_tab n LEFT JOIN txt_tab t ON n.thekey =
> t.thekey AND t.class = class_tab.tkey AND n.class = class_tab.class
> WHERE n.thekey < 5;
> produces:
> NOTICE: Adding missing FROM-clause entry for table "class_tab"
> ERROR: JOIN/ON clause refers to "class_tab", which is not part of JOIN
>
> So how do I do this?

I think you want something like:

select distinct n.thekey, n.val, t.txt FROM class_tab JOIN num_tab n
using (class) LEFT JOIN txt_tab t on (t.thekey=n.thekey and t.class =
class_tab.tkey);