Re: [Newbie] UPDATE based on other table content

2009/10/29 Daniel Chiaramello <daniel(dot)chiaramello(at)golog(dot)net>:
> Never mind, I found how finally:
>
> UPDATE
>   product
> SET
>   qty = qty+s_count
> FROM (
>       SELECT
>           intermediate.product_id,
>           count(*) AS s_count
>       FROM
>           intermediate,
>           orders
>       WHERE
>           orders.intermediate_id=intermediate.id
>       GROUP BY
>           intermediate.product_id
>   ) AS summary
> WHERE
>   summary.product_id = product.id
> ;
>
> Sorry for disturbance!
> Daniel
>
> Daniel Chiaramello a écrit :
>>
>> Hello.
>>
>> I have a very basic question, relative to the following "problem".
>>
>> I have the following tables:
>>
>> product
>>   id
>>   qty
>>
>> intermediate
>>   id
>>   product_id
>>
>> orders
>>   intermediate_id
>>
>> I want to update the "qty" field of the "product" table by incrementing it
>> each time there is an order in the "orders" table, referencing a given
>> product through the "intermediate" table.
>>
>> I tried the following request:
>>
>> UPDATE
>>       qty = qty+1
>> FROM
>>   intermediate,
>>   orders
>> WHERE
>>   orders.intermediate_id=intermediate.id AND
>>   intermediate.product_id=product.id
>> ;
>>
>> But of course it does what was predictable - ie the qty "field" is
>> incremented only once, even if more than one entry is referencing a given
>> product. But it's not what I was hoping...
>>
>> What would be the "good" solution to do that UPDATE?
>>
>> Thanks for your attention!
>> Daniel Chiaramello
>>
>

Your solution looks like it would perform a cumulative calculation.
Surely you'd want qty = s_count?

In any case, wouldn't you be better off not having a quantity column
at all and just calculate it with either a query or a view?

Example:

SELECT product.id, COUNT(intermediate.product_id)
FROM product
LEFT JOIN intermediate ON product.id = intermediate.product_id
GROUP BY product.id
ORDER BY product.id

Or include a product name in the product table to get more meaningful
output. You'd then get an output like:

id name count
1 Orange 5
2 Apples 7
3 Pears 2
4 Kiwi 0

If you don't want ones for which there have been no orders for (or
whatever your intermediate table is for), use an INNER JOIN instead.

Regards

Thom