Re: table name "unnest" specified more than once

On Thursday, February 27, 2020, Guyren Howe <guyren(at)gmail(dot)com> wrote:

> # select
> *
> from
> unnest(array[array['a', 'b'], array['c', 'c']]),
> unnest(array[array['1', '2'], array['3', '4']]);
> ERROR: 42712: table name "unnest" specified more than once
>

This specific error is resolved by using the alias feature of the FROM
clause:

alias

A substitute name for the FROM item containing the alias. An alias is used
for brevity or to eliminate ambiguity for self-joins (where the same table
is scanned multiple times)

>
> I’m trying to cross-join multiple two-dimensional arrays, expecting to
> retain the inner arrays. I’ve been trying for hours without luck; the
> ever-esoteric SQL syntax foils me at every turn.
>
> It’s a shame I can’t get the unnest function not to just concatenate the
> inner arrays if I just put a bunch of them. This doesn’t strike me as
> optimal behavior.
>

Putting unnest in a select-list might get you closer to your goal. Or the
array-array concatenating operator.

>
> For more context, I’m trying to make a system of functions to score a
> Texas Hold ‘Em game. So I have a card type consisting of a pair of suit and
> rank, and I’m tossing them about. The cross-join is so I can build all
> candidate hands for scoring. I’m trying to create a function I can call
> like this:
>
> select
> best_hands_with_river(
> array[
> c('H', 'K'),
> c('D', 'A')
> ],
> array[
> c('C', '2'),
> c('C', 'K'),
> c('S', 'K'),
> c('H', 'A'),
> c('C', 'A')
> ])
>
> Here, c is a function that constructs a card type. Card is a ROW(varchar,
> varchar).
>
> So: how do I cross-join three identical arrays of my card type?
>

Something like:

Select f1.a, f2.b
From (select * from unnest(arr)) as f1 (a)
Cross join (select * From unnest(arr)) as f2 (b)
Etc...Maybe with parentheses...

There may be a more succinct way to write this but going verbose until it
works minimizes the amount of syntax you need to deal with. Though I
personally encourage writing intentional cross join (really, all joins)
using join syntax instead of comma-separated from items.

David J.