[tempguy9999]

This is a pretty trivial list. Useful for beginners I guess.

I seriously take issue with "Reference Column Position in GROUP BY and ORDER BY" though. If it is restricted to ad-hoc (AKA messing-about) queries I'd be fine with it, but it won't be. Just don't do it.

[wfriesen]

It's especially egregious in the ORDER BY, since there you have the option of using column aliases.

[commandlinefan]

Are you saying you can't use column aliases in group by? What version of Postgres are you using? I just tried it in 11.5 and it worked:

    # select cust_id as c, sum(avail_balance) as b from account group by c order by b;

[wfriesen]

I was thinking of Oracle, where aliases are evaluated at column protection time, so after grouping but before ordering.

[tempguy9999]

Interesting. It doesn't work in MSSQL, and I understand that's correct (ie. isn't allowed) per the standard.

[commandlinefan]

Huh - I guess I never thought about it. It makes sense to disallow it, though - column aliases are there to rename complex expressions, which you probably _shouldn't_ be grouping on anyway.

[yellowapple]

> which you probably _shouldn't_ be grouping on anyway.

This is frequently unavoidable, though. Or more precisely: it could be avoided with a sane database design, but the databases on which I have to work for my day job are the precise opposite of "well-designed", so grouping on complex expressions is unfortunately an inevitability.

[commandlinefan]

That's true - I can definitely imagine having to group on something like "concat(lastname + ', ' + firstname)".

[tempguy9999]

I think it's for other reasons (and grouping on expressions is quite reasonable anyway).

It's (IIRC!) something to do with the situation of

  select x + y as x 
  from ...
  group by x

which x are we talking about? (Logically that example is crap because only the alias x makes sense, but something like that anyway).

[tempguy9999]

I always, always forgot what column aliases I can use where. Thanks for the reminder.

[irrational]

It's useful for people new to Postgres, since many of these things are particular to Postgres.

[tempguy9999]

Not really. Most is pretty well standard SQL (CTE optimisation fence pre PG12 being one exception, and there are a couple more, but really it's mostly standard stuff).

[barrkel]

The :: syntax for CAST() is also a psql-ism.

[tejtm]

Maybe that is what it is now, but I still have muscle memory using it in Informix.

[irrational]

Maybe. I come from the Oracle world and the majority of these don't apply to Oracle.