mysql - 如何在 SQL 中搜索并获取大表的计数(where-in)

Question

我有sql表:

id  user    city
1   Alisa   New York
2   Alisa   Sydney
3   Alisa   Rom
4   Alisa   Toronto
5   Bob     Rom
6   Bob     Moskow
7   Bob     Sydney
8   Tom     Sydney

例如，我需要了解有多少个相同城市有 Bob 的其他用户。

SELECT user, count(DISTINCT city) FROM table WHERE city IN (
   SELECT city FROM table WHERE user = 'Bob'
) AND user != 'Bob' GROUP BY user

结果是:

user    count(DISTINCT city)
Alisa   2
Tom     1

您知道提出此请求的其他最佳方式吗？ 大 table 可以吗？

Answer 1

这个怎么样:

SELECT A.user, 
       count(DISTINCT A.city)
FROM table  A,
     (SELECT city FROM table WHERE user = 'Bob') B
WHERE A.city = B.city
AND A.user != 'Bob'
GROUP BY A.user