Question

我可以以某种方式连接表并避免在以下MySQL查询中使用distinct。 invite_by_id显示邀请此用户的用户ID。

SELECT
    user1.id, count(distinct user2.id) AS theCount, count(distinct user3.id) AS theCount2
FROM
    users AS user1
LEFT OUTER JOIN
    users AS user2 ON user2.invited_by_id=user1.id
LEFT OUTER JOIN (
    SELECT id, invited_by_id FROM users WHERE signup_date >= NOW() - INTERVAL 30 DAY
) AS user3 ON user3.invited_by_id=user1.id
GROUP BY user1.id;

Answer 1

我在此假设您正在尝试计算用户被邀请的次数以及该用户在过去30天内被邀请的次数。

在这种情况下，您可以使用简单的条件和进行查询：

select user1.id, count(user2.id) as tehCount, sum(user2.signup_date >= NOW() - INTERVAL 30 DAY) as theCount2
from users as user1
left outer join users as user2 on user2.invited_by_id = user1.id
group by user1.id

如果theCount2中的空值出现问题，请使用coalesce：

coalesce(sum(user2.signup_date >= NOW() - INTERVAL 30 DAY), 0)

Answer 2

如果您运行的MySQL版本大于5.0.37，则可以使用Profiler，这可以让您非常了解任何查询的瓶颈所在。这可能是一个很好的起点 - 如果您不确定如何最好地解释输出，您可以将输出编辑为原始问题。

Answer 3

尝试这样的事情，我更改了子查询表名称，使其更清晰：

Select
    user.id,
    all_time.total AS theCount, 
    last_month.total AS theCount2
From users AS user
Left Outer Join 
    (Select Count(id) as total, invited_by_id
     From users
     Group By invited_by_id) as all_time
       On all_time.invited_by_id = user.id
Left Outer Join
    (Select Count(id) as total, invited_by_id
     From users 
     Where signup_date >= NOW() - INTERVAL 30 DAY
     Group By invited_by_id) AS last_month 
       On last_month.invited_by_id = user.id

如果这是您经常运行的内容，请确保已将user.invited_by_id编入索引！

如何优化嵌套查询？

3 个答案: