即使使用 DISTINCT，MYSQL 也会加入重复的结果

Question

我有 2 个 mysql 表，一个称为“用户”，另一个称为“连接”

表用户有关于用户的信息并且有 3 行：

mikha
guy
maricela

Table connections 有 twitter 等用户之间的连接（例如 mikha 关注 maricela 和 maricela 关注 mikha）

Connections 有以下几行：

username1     | username2
--------------------------
guy           | maricela
mikha         | guy
mikha         | maricela

我想获取有关“mikha”的信息，包括他关注的人数和关注他的人数。

我使用以下查询：

SELECT *, COUNT(DISTINCT  connections.username1) AS count_following, 
   COUNT(DISTINCT connections.username2)  AS count_followers 
FROM users LEFT JOIN connections on connections.username1 = 'mikha' OR  
   connections.username2 = 'mikha' WHERE users.username = 'mikha'

预期：

count_following = 2 (as mikha is following guy and maricela)
count_followers = 0 (no one following mikha yet)

实际：

count_following = 2
count_followers = 1

谢谢 问候 迈克尔

Answer 1

您可以运行两个不同的查询 (fiddle here)：

select
  (select count(*) from connections
  where username1 = 'mikha') following,
  (select count(*) from connections
  where username2 = 'mikha') followers

或者直接使用这个 (fiddle here)：

select
    sum(username1 = 'mikha') following,
    sum(username2 = 'mikha') followers
from connections

我认为第二个应该更快。一旦你得到它，只需加入你的用户表，以防你需要来自该表的额外信息（根据你的例子，你不需要）。