subselects

> Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> writes:
> >> Given
> >> create table t1 (name text, value float8);
> >>
> >> this fails:
> >> SELECT name, value FROM t1 as touter WHERE
> >> (value/(SELECT AVG(value) FROM t1 WHERE name = touter.name)) > 0.75;
> >> ERROR: parser: '/' must return 'bool' to be used with subquery
>
> > Aren't you really saying WHERE col / (subselect). That doesn't return
> > bool, so the message seems correct.
>
> No, look again: the result of the subselect is being used as an operand
> within the WHERE clause:
> WHERE (value/(SUBSELECT)) > 0.75;
>
> If the / were the toplevel operator in the WHERE then the message would
> make sense, because the WHERE clause as a whole must yield boolean.
> But that doesn't mean that the operator immediately above the subselect
> must yield boolean.
>
> Besides, I already fixed this ;-)
>
> > We don't allow subselects in target lists, or inside expressions.
>
> We don't allow 'em in target lists, I think (anyone understand why not?)
> but they work fine inside expressions in WHERE or HAVING.

Gee, I didn't know they worked inside an expression, but now looking
at the grammar, I remember seeing that they should.

Added to TODO list.

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