Question

我有一个表用户：

user_id    name
1          John
2          Dan
3          Jane
4          Sophie
5          Jodie

然后我有一个名为associates的表：

user_id    assoc_id
1          2
1          3 
3          4
3          1
3          5
4          1
5          1
5          2

我想要做的是显示每个用户有多少个员工，或者没有

所以，结果会显示

user_id    Name     Number of Associates
1          John     2
2          Dan      0
3          Jane     3
4          Sophie   1
5          Jodie    2

我正在尝试的是什么，但没有显示那些0

这是我正在尝试的，我如何获得0？

SELECT u.user_id, u.name, count(a.user_id) as howmany from users u
join associates a on a.user_id = u.user_id
group by u.user_id order by u.user_id asc

Answer 1

你需要LEFT OUTER JOIN：

SELECT u.user_id, u.name, count(a.user_id) as howmany 
FROM users u LEFT OUTER JOIN associates a 
ON a.user_id = u.user_id
GROUP BY u.user_id 
ORDER BY u.user_id ASC

LEFT OUTER JOIN将包括第一个表的每一行，即使它们不在连接表中。

Answer 2

试试这个：

    SELECT u.user_id, 
    u.name, 
    count(a.user_id) as howmany 
    FROM users u 
    LEFT OUTER JOIN associates a 
    ON a.user_id = u.user_id
    GROUP BY u.user_id 
    ORDER BY u.user_id ASC

Answer 3

尝试使用左连接而不是内连接，这应该选择没有相应关联的行。

SELECT u.user_id, u.name, count(a.user_id) as howmany 
from users u
left join associates a 
  on a.user_id = u.user_id
group by u.user_id 
order by u.user_id asc

Answer 4

这也适用于严格的规则（如果我没有做过任何错字）：

SELECT   users.*, ISNULL(associatesCount.AssocCount, 0) AS AssocCount
FROM    users
LEFT JOIN (
        SELECT user_id, COUNT(*) AS AssocCount
        FROM associates
        GROUP BY user_id
        ) AS associatesCount
        ON users.user_id = associatesCount.user_id

表中的双重计数

4 个答案: