Re: COALESCE function

Hello All,

Attempting to select two different column types with COALESCE returns
this error:

ERROR: COALESCE types smallint and character varying cannot be matched

Attempting the same thing with a CASE statement returns a similar error:

ERROR: CASE types smallint and character varying cannot be matched

I also checked Oracle's NVL command, and it throws a similar error.
Clearly, I'm not supposed to intermix two different column types into a
SELECT statement. This is because the engine needs to return a set
given the criteria, and it's difficult to do that with criteria that
/depends on the data/, yes?

Could someone explain a/the more formal reason why I can't do what I'm
trying to do?

The relevant part of my SELECT statement:

SELECT
...,
COALESCE(f.number, f.name),
...
FROM
...,
field AS f,
...
WHERE
...
;

f.number ∈ SMALLINT
f.name ∈ CHARACTER VARYING

Thank you in advance!

Kevin

P.S. If something gets lost in bit/encoding translation, ∈ = "Element Of"

Based on the below each row could end up returning a different data type
compared to a previous row for that column.

SELECT COALESCE( CAST(f.number as varchar(100)) , f.name) FROM....

Whatever f.name is set to in terms of the max length of varchar, if any,
is what f.number should be cast to.

mike

On Sun, 2006-12-31 at 00:44 -0500, Kevin Hunter wrote:
> Hello All,
>
> Attempting to select two different column types with COALESCE returns
> this error:
>
> ERROR: COALESCE types smallint and character varying cannot be matched
>
> Attempting the same thing with a CASE statement returns a similar error:
>
> ERROR: CASE types smallint and character varying cannot be matched
>
> I also checked Oracle's NVL command, and it throws a similar error.
> Clearly, I'm not supposed to intermix two different column types into a
> SELECT statement. This is because the engine needs to return a set
> given the criteria, and it's difficult to do that with criteria that
> /depends on the data/, yes?
>
> Could someone explain a/the more formal reason why I can't do what I'm
> trying to do?
>
> The relevant part of my SELECT statement:
>
> SELECT
> ...,
> COALESCE(f.number, f.name),
> ...
> FROM
> ...,
> field AS f,
> ...
> WHERE
> ...
> ;
>
> f.number ∈ SMALLINT
> f.name ∈ CHARACTER VARYING
>
> Thank you in advance!
>
> Kevin
>
> P.S. If something gets lost in bit/encoding translation, ∈ = "Element Of"
>
>
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