tricky GROUP BY / JOIN question

Hello,
I'm sorry I couldn't think up a more precise subject line.

I would like to know whether the following can be implemented in SQL:
The example below joins 4 tables ITEM, BRAND, MODEL and CONDITION. In
human understandable terms: a [secondhand] Item is of a particular Model
and Brand. The Items retail at different prices depending on their
Condition.

Required result set:

Brand | Model | Cond | Cond | Price | Price
| | min | max | min | max
-------------------------------------------
Canon | A1 | Exc | Mint | 139 | 155
Canon | F1N | Exc++| Mint-| 329 | 379
Canon | 24mm | Exc--| Mint+| 99 | 179
Nikon | 50mm | Exc--| Mint+| 109 | 119


This is *almost* what I need:

SELECT
BRAND.BRAND_NAME,
MODEL.MODEL_NAME,
min (ITEM.PRICE),max (ITEM.PRICE)
*min (CONDITION.POSITION),max (CONDITION.POSITION)*
FROM ITEM
left outer join MODEL on MODEL_PK =ITEM.MODEL_FK
left outer join BRAND on BRAND.BRAND_PK =MODEL.BRAND_FK
left outer join CONDITION on CONDITION.CONDITION_PK = ITEM.CONDITION_FK
group by BRAND.BRAND_NAME,MODEL.MODEL_NAME

Problem:
Table CONDITION has the columns
- NAME varchar(5)
- POSITION int2

In the result I don't want min/max(POSITION) but CONDITION.NAME of min
and max(POSITION) for each MODEL.

Is this possible at all?
--


Regards/Gruß,

Tarlika Elisabeth Schmitz

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Re: tricky GROUP BY / JOIN question

T E Schmitz writes:
> This is *almost* what I need:

> SELECT
> BRAND.BRAND_NAME,
> MODEL.MODEL_NAME,
> min (ITEM.PRICE),max (ITEM.PRICE)
> *min (CONDITION.POSITION),max (CONDITION.POSITION)*
> FROM ITEM
> left outer join MODEL on MODEL_PK =ITEM.MODEL_FK
> left outer join BRAND on BRAND.BRAND_PK =MODEL.BRAND_FK
> left outer join CONDITION on CONDITION.CONDITION_PK = ITEM.CONDITION_FK
> group by BRAND.BRAND_NAME,MODEL.MODEL_NAME

> In the result I don't want min/max(POSITION) but CONDITION.NAME of min
> and max(POSITION) for each MODEL.

I think you could do something like

SELECT
BRAND.BRAND_NAME,
MODEL.MODEL_NAME,
min (ITEM.PRICE),max (ITEM.PRICE)
(select name from condition c1 where position = min(condition.position)),
(select name from condition c2 where position = max(condition.position)),
FROM ITEM
left outer join MODEL on MODEL_PK =ITEM.MODEL_FK
left outer join BRAND on BRAND.BRAND_PK =MODEL.BRAND_FK
left outer join CONDITION on CONDITION.CONDITION_PK = ITEM.CONDITION_FK
group by BRAND.BRAND_NAME,MODEL.MODEL_NAME

ie do a sub-select to get the desired name.

You need Postgres 7.4 or later to get this to work --- before that we
would have mis-interpreted the aggregate calls to indicate aggregation
within the sub-selects. The current interpretation is per SQL spec:
since the aggregate argument is a variable of the outer select, the
aggregation occurs with respect to that select, and the aggregate result
is passed down to the sub-select as a scalar.

regards, tom lane

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Re: tricky GROUP BY / JOIN question

Hello Tom,

Tom Lane wrote:

> T E Schmitz writes:
>
>>This is *almost* what I need:
>
>
>>SELECT
>>BRAND.BRAND_NAME,
>>MODEL.MODEL_NAME,
>>min (ITEM.PRICE),max (ITEM.PRICE)
>>*min (CONDITION.POSITION),max (CONDITION.POSITION)*
>>FROM ITEM
>>left outer join MODEL on MODEL_PK =ITEM.MODEL_FK
>>left outer join BRAND on BRAND.BRAND_PK =MODEL.BRAND_FK
>>left outer join CONDITION on CONDITION.CONDITION_PK = ITEM.CONDITION_FK
>>group by BRAND.BRAND_NAME,MODEL.MODEL_NAME
>
>
>>In the result I don't want min/max(POSITION) but CONDITION.NAME of min
>>and max(POSITION) for each MODEL.
>
>
>
> (select name from condition c1 where position = min(condition.position)),
> (select name from condition c2 where position = max(condition.position)),
>

Thank you, Tom, this worked a treat!

--


Regards/Gruß,

Tarlika Elisabeth Schmitz

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