Question

许多问题也被问到相同的问题。但是先听我说。我有两个使用LEFT JOIN连接的表，第一个表是用户表，第二个表只是引用表划分表

用户表

 user_id |  name   |   division_id
 0       |  ABC    |     0
 1       |  DEF    |     1
 2       |  GHI    |     1

分区表

id  |  name   | protect class
1   |  div_1  |      1

我的原始查询

SELECT count(usr.user_id) 
FROM user as usr 
    LEFT JOIN division as dv 
        ON dv.id = usr.division_id
WHERE 
    dv.protect_class != '1'

但是当我想选择不带分隔符的用户

SELECT count(usr.user_id) 
    FROM m_users as usr 
        LEFT JOIN m_division as dv 
            ON dv.id = usr.division_id OR  usr.division_id = '0'
    WHERE 
         dv.protect_class != '1'

事情有点怪异，不具有Division_id或等于0的用户将根据获得的除法条目数进行多次计数。

我也尝试添加独特的

SELECT distinct count(usr.user_id)
    FROM m_users as usr 
        LEFT JOIN m_division as dv 
            ON dv.id = usr.division_id OR usr.division_id = '0'
    WHERE 
        dv.protect_class != '1'

但是对我来说仍然没有运气。当我查询这个

SELECT distinct usr.*
    FROM m_users as usr 
        LEFT JOIN m_division as dv 
            ON dv.id = usr.division_id OR usr.division_id = '0'
    WHERE 
        dv.protect_class != '1'

然后对它进行物理计数（手动），计数是正确的，那么如果我应用count（）函数，则计数是错误的。所以我只想请教一下，如何计算条目而不计算我的LEFT JOIN中的重复项？

Answer 1

您可以在下面尝试-count(distinct usr.user_id)

SELECT count(distinct  usr.user_id)
    FROM m_users as usr 
        LEFT JOIN m_division as dv 
            ON dv.id = usr.division_id OR usr.division_id = '0'
    WHERE 
        dv.protect_class != '1'

Answer 2

以“ OR”联接会在匹配条件时导致多次联接

SELECT count(usr.user_id) 
    FROM m_users as usr 
    LEFT JOIN m_division as dv 
         ON dv.id = usr.division_id
    WHERE 
         dv.protect_class != '1'
         and (usr.division_id = '0' OR usr.division_id IS NULL)

带有LEFT JOIN的MySQL count（）

2 个答案: