MySQL将多个子查询优化为嵌套内部联接？

我有三张桌子；合同、经销商和用户

用户有许多经销商，经销商有许多合同，但这些合同与用户没有直接关联

我试图建立一个报告，让我每月完成的合同计数按用户分组过去12个月

到目前为止，我已经构建了一个多个子查询，速度非常慢：

我不想使用子查询来选择每个月，而是希望使用如下内容：

COUNT(*) as last_12_months,
SUM(case when MONTH(completion_date) = 8 then 1 else 0 end) as Aug_2012,
SUM(case when MONTH(completion_date) = 9 then 1 else 0 end) as Sep_2012,
etc.

因为我将返回多个列，所以我必须对其进行重组，但我不确定如何进行重组。如果我使用内部连接，我应该在什么子句上连接

以下是基于米哈伊尔回答的最终查询： 这大约快4倍。

试试这个：

select
    User.id, User.name,
    sum(case when MONTH(completion_date) = 8 and Year(completion_date)=2012 then 1 else 0 end) as Aug_2012,
    sum(case when MONTH(completion_date) = 9 and Year(completion_date)=2012 then 1 else 0 end) as Sep_2012,
    sum(case when MONTH(completion_date) = 10 and Year(completion_date)=2012 then 1 else 0 end) as Oct_2012,
    sum(case when MONTH(completion_date) = 11 and Year(completion_date)=2012 then 1 else 0 end) as Nov_2012,
    sum(case when MONTH(completion_date) = 12 and Year(completion_date)=2012 then 1 else 0 end) as Dec_2012,
    sum(case when MONTH(completion_date) = 1 and Year(completion_date)=2013 then 1 else 0 end) as Jan_2012,
    sum(case when MONTH(completion_date) = 2 and Year(completion_date)=2013 then 1 else 0 end) as Feb_2012,
    sum(case when MONTH(completion_date) = 3 and Year(completion_date)=2013 then 1 else 0 end) as Mar_2012,
    sum(case when MONTH(completion_date) = 4 and Year(completion_date)=2013 then 1 else 0 end) as Apr_2012,
    sum(case when MONTH(completion_date) = 5 and Year(completion_date)=2013 then 1 else 0 end) as May_2012,
    sum(case when MONTH(completion_date) = 6 and Year(completion_date)=2013 then 1 else 0 end) as Jun_2012,
    sum(case when MONTH(completion_date) = 7 and Year(completion_date)=2013 then 1 else 0 end) as Jul_2012
from users AS User
left outer join dealers on
    User.id=dealers.user_id
left outer join contracts on
    dealers.id=contracts.dealer_id
group by
    User.id,
    contracts.status
having
    contracts.status='Paid'
order by
    User.name asc;

SELECT
User.*,
SUM(case when MONTH(completion_date) = 8 then 1 else 0 end) AS Aug_2012,
SUM(case when MONTH(completion_date) = 9 then 1 else 0 end) AS Sep_2012,
SUM(case when MONTH(completion_date) = 10 then 1 else 0 end) AS Oct_2012,
SUM(case when MONTH(completion_date) = 11 then 1 else 0 end) AS Nov_2012,
SUM(case when MONTH(completion_date) = 12 then 1 else 0 end) AS Dec_2012,
SUM(case when MONTH(completion_date) = 1 then 1 else 0 end) AS Jan_2013,
SUM(case when MONTH(completion_date) = 2 then 1 else 0 end) AS Feb_2013,
SUM(case when MONTH(completion_date) = 3 then 1 else 0 end) AS Mar_2013,
SUM(case when MONTH(completion_date) = 4 then 1 else 0 end) AS Apr_2013,
SUM(case when MONTH(completion_date) = 5 then 1 else 0 end) AS May_2013,
SUM(case when MONTH(completion_date) = 6 then 1 else 0 end) AS Jun_2013,
SUM(case when MONTH(completion_date) = 7 then 1 else 0 end) AS Jul_2013,
SUM(case when completion_date BETWEEN '2012-08-01 00:00:00' AND '2013-07-31 23:59:59' then 1 else 0 end) as last_12_months
FROM users AS User
LEFT OUTER JOIN 
( 
    SELECT id, user_id FROM dealers 
    WHERE active = 1 AND user_id IS NOT NULL
) AS Dealer ON User.id = Dealer.user_id
LEFT OUTER JOIN
(
    SELECT completion_date, status, dealer_id FROM contracts
    WHERE completion_date BETWEEN '2012-08-01 00:00:00' AND '2013-07-31 23:59:59' AND status = 'Paid' AND cancelled = 0
) AS Contract on Dealer.id = Contract.dealer_id
WHERE
    User.id IN 
    (
        SELECT user_id FROM dealers
        WHERE active = 1 AND user_id IS NOT NULL
        GROUP BY user_id
    )
GROUP BY
    User.id order by User.name asc

select
    User.id, User.name,
    sum(case when MONTH(completion_date) = 8 and Year(completion_date)=2012 then 1 else 0 end) as Aug_2012,
    sum(case when MONTH(completion_date) = 9 and Year(completion_date)=2012 then 1 else 0 end) as Sep_2012,
    sum(case when MONTH(completion_date) = 10 and Year(completion_date)=2012 then 1 else 0 end) as Oct_2012,
    sum(case when MONTH(completion_date) = 11 and Year(completion_date)=2012 then 1 else 0 end) as Nov_2012,
    sum(case when MONTH(completion_date) = 12 and Year(completion_date)=2012 then 1 else 0 end) as Dec_2012,
    sum(case when MONTH(completion_date) = 1 and Year(completion_date)=2013 then 1 else 0 end) as Jan_2012,
    sum(case when MONTH(completion_date) = 2 and Year(completion_date)=2013 then 1 else 0 end) as Feb_2012,
    sum(case when MONTH(completion_date) = 3 and Year(completion_date)=2013 then 1 else 0 end) as Mar_2012,
    sum(case when MONTH(completion_date) = 4 and Year(completion_date)=2013 then 1 else 0 end) as Apr_2012,
    sum(case when MONTH(completion_date) = 5 and Year(completion_date)=2013 then 1 else 0 end) as May_2012,
    sum(case when MONTH(completion_date) = 6 and Year(completion_date)=2013 then 1 else 0 end) as Jun_2012,
    sum(case when MONTH(completion_date) = 7 and Year(completion_date)=2013 then 1 else 0 end) as Jul_2012
from users AS User
left outer join dealers on
    User.id=dealers.user_id
left outer join contracts on
    dealers.id=contracts.dealer_id
group by
    User.id,
    contracts.status
having
    contracts.status='Paid'
order by
    User.name asc;