On Wed, Oct 20, 2021 at 6:58 AM Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
"David G. Johnston" <david.g.johnston@xxxxxxxxx> writes:
> On Tuesday, October 19, 2021, Michael Lewis <mlewis@xxxxxxxxxxx> wrote:
>> On Tue, Oct 19, 2021 at 3:48 PM Mithran Kulasekaran <
>> mithranakulasekaran@xxxxxxxxx> wrote:
>>> create view template_view (id, name, description, is_staged) as
>>> select t.id,t.name, t.description, false as is_staged
>>> from template t
>>> left join template_staging ts on t.name = ts.name and ts.name is null
>> Does that work? I've only seen that type of logic written as-
>> left join template_staging ts on t.name = ts.name
>> where ts.name is null
> The are functionally equivalent, though the timing of the _expression_
> evaluation differs slightly.
No, not at all. Michael's version correctly implements an anti-join,
where the first version does not. The reason is that the WHERE clause
"sees" the column value post-JOIN, whereas the JOIN/ON clause "sees"
values pre-JOIN.
Yeah, my bad. I was actually thinking this but then figured the OP wouldn't have written an anti-join that didn't actually work.
My original email was going to be:
Adding the single table _expression_ to the ON clause is shorthand for writing:
SELECT t.* FROM template AS t LEFT JOIN (SELECT * FROM template_staging WHERE template_staging.name IS NULL) AS ts ON t.name = ts.name;
David J.
