Обсуждение: Re: [HACKERS] "op must return bool to be used with subquery"?

> 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.

We don't allow subselects in target lists, or inside expressions.

> 
> The code in parse_expr.c that produces this message claims to be
> enforcing that "sub-selects can only be used in WHERE clauses".
> Either the comment is inaccurate or the test is too restrictive.
> If the test is correct then I think the error message is unhelpful.
> Anybody understand this code well enough to know what's really going on?
> 
>             regards, tom lane
> 
> 


--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
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Bruce Momjian <maillist@candle.pha.pa.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.
        regards, tom lane