Re: Strange behavior on non-existent field in subselect?

On þri, 2006-10-17 at 15:58 -0700, Ken Tanzer wrote:
> We're a little puzzled by this (apparently) strange behavior, and would
> be curious to know what you folks make of it. Thanks.

not sure exactly what you are referring to, but:
(rearranged quotes to group output with SQL)

> SELECT foo_field FROM par;
> psql:strangefield.sql:11: ERROR: column "foo_field" does not exist

hopefully, no mystery here.

> SELECT foo_field FROM foo WHERE foo_field IN (SELECT foo_field FROM par);

if par is empty, then this SELECT will return 0 rows,
otherwise it is equivalent to SELECT foo_field from foo

> foo_field
> -----------
> (0 rows)

foo is empty, so no rows returned

> INSERT INTO foo VALUES (1);
> SELECT foo_field FROM foo WHERE foo_field IN (SELECT foo_field FROM par);
> foo_field
> -----------
> (0 rows)

par is empty, so the IN operator fails for the foo row

> INSERT INTO par VALUES (1);
> SELECT foo_field FROM foo WHERE foo_field IN (SELECT foo_field FROM par);
> foo_field
> -----------
> 1
> (1 row)

when par contains at least one row, the subselect will
return foo_field once per row of par.
the IN operator will ignore duplicates, so the result
is the same for any number of rows in par greater than 0

gnari