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Thread: "Subquery must return only one column" & query optimization

"Subquery must return only one column" & query optimization

From
"Philippe Lang"
Date:
Hi,

I was trying to run this query this morning:

--------------------------
SELECT

r.*,

(SELECT
rl.reminder_header,rl.reminder_footer
FROM reminder_levels AS rlWHERE rl.lookup =(    SELECT MAX(reminder_level_lookup)     FROM reminders    WHERE
customer_id= r.customer_id) 
)

FROM reminders AS r
--------------------------

Postgresql replied that:

--------------------------
ERROR: subquery must return only one column
SQL state: 42601
--------------------------

Is there a way to avoid writing:

--------------------------
SELECT

r.*,

(SELECT
rl.reminder_header
FROM reminder_levels AS rlWHERE rl.lookup =(    SELECT MAX(reminder_level_lookup)     FROM reminders    WHERE
customer_id= r.customer_id) 
) AS reminder_header,

(SELECT
rl.reminder_footer
FROM reminder_levels AS rlWHERE rl.lookup =(    SELECT MAX(reminder_level_lookup)     FROM reminders    WHERE
customer_id= r.customer_id) 
) AS reminder_footer

FROM reminders AS r
--------------------------

... which works, but runs twice the same subselect block:

--------------------------FROM reminder_levels AS rlWHERE rl.lookup =(    SELECT MAX(reminder_level_lookup)     FROM
reminders   WHERE customer_id = r.customer_id) 
--------------------------

Thanks,

Philippe Lang

Re: "Subquery must return only one column" & query optimization

From
Tom Lane
Date:
"Philippe Lang" <philippe.lang@attiksystem.ch> writes:
> I was trying to run this query this morning:

> --------------------------
> SELECT 

> r.*,

> (
>     SELECT 

>     rl.reminder_header,
>     rl.reminder_footer

>     FROM reminder_levels AS rl
>     WHERE rl.lookup =
>     (
>         SELECT MAX(reminder_level_lookup) 
>         FROM reminders
>         WHERE customer_id = r.customer_id
>     )
> )

> FROM reminders AS r
> --------------------------

> Postgresql replied that:

> --------------------------
> ERROR: subquery must return only one column

Since 8.0 or so you could write the sub-select as
SELECT ROW(rl.reminder_header, rl.reminder_footer) FROM ...

We ought to make that happen automatically, but it's not real
high on the to-do list.
        regards, tom lane

Re: "Subquery must return only one column" & query optimization

From
"Philippe Lang"
Date:
pgsql-sql-owner@postgresql.org wrote:
> "Philippe Lang" <philippe.lang@attiksystem.ch> writes:
>> I was trying to run this query this morning:
>
>> --------------------------
>> SELECT
>
>> r.*,
>
>> (
>>     SELECT
>
>>     rl.reminder_header,
>>     rl.reminder_footer
>
>>     FROM reminder_levels AS rl
>>     WHERE rl.lookup =
>>     (
>>         SELECT MAX(reminder_level_lookup)
>>         FROM reminders
>>         WHERE customer_id = r.customer_id
>>     )
>> )
>
>> FROM reminders AS r
>> --------------------------
>
>> Postgresql replied that:
>
>> --------------------------
>> ERROR: subquery must return only one column
>
> Since 8.0 or so you could write the sub-select as
>
>     SELECT ROW(rl.reminder_header, rl.reminder_footer) FROM ...
>
> We ought to make that happen automatically, but it's not real high on
> the to-do list.

Hi Tom,

Fine, the query is faster now:

------------------------
SELECT

r.*,

(SELECT
ROW(rl.reminder_header, rl.reminder_header)
FROM reminder_levels AS rlWHERE rl.lookup =(    SELECT MAX(reminder_level_lookup)     FROM reminders    WHERE
customer_id= r.customer_id) 
) AS rec

FROM reminders AS r
------------------------

... but the last column is now of type "record", and since this query
serves as a datasource for a MS Access report, it is not able to "split"
the record into the values that interest me: reminder_header and
reminder_footer!

Regards,

Philippe Lang