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
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