Re: Whats the most efficient query for this result?

Hi Nick,

On 17/01/12 00:18, Nick wrote:
> I have three tables (users, books, pencils) and would like to get a
> list of all users with a count and total price of their books and
> pencils for 2012-01-01...
>
> So with this data...
>
> users (user_id)
> 1
> 2
> 3
>
> books (user_id, price, created)
> 1 | $10 | 2012-01-01
> 1 | $10 | 2012-01-01
> 3 | $10 | 2012-01-01
>
> pencils
> 1 | $.50 | 2012-01-02
> 3 | $.50 | 2012-01-01
> 3 | $.50 | 2012-01-02
>
> What is the most efficient way to get this result...
>
> query_result (user_id, book_count, book_price_total, pencil_count,
> pencil_price_total)
> 1 | 2 | $20 | 0 | $0
> 2 | 0 | $0 | 0 | $0
> 3 | 1 | $10 | 1 | $.50
>
>

Seems straightforward enough - left join the tables, group the result on
user_id - so I'd write it as:

select u.user_id, count(b.user_id) as "book_count",
coalesce(sum(b.price), 0) as "book_price_total", count(p.user_id) as
"pencil_count", coalesce(sum(b.price), 0) as "pencil_price_total"
from users u
left join books b on b.user_id = u.user_id and b.created = '2012-01-01'
left join pencils p on p.user_id = u.user_id and p.created = '2012-01-01'
group by u.user_id
order by u.user_id

If you need something more efficient, summary tables may help - hard to
say without knowing more about the real data.

Tom