Question

我有两个表组和联系人，我在联系表中获取所有组的联系人总数。就像在组表中一样，我有值

  group1
  group2

在联系表中我有

  myname group1
  myname1 group1
  myname2 group1

现在我想要所有的groupname与他们的联系人数量

  group1 3
  group2 0

我用过：

SELECT g.gid,g.groupname,g.TYPE,g.DATE,COUNT(*)AS cnt,1 
FROM groupname g,contacts c 
WHERE g.gid=c.gid AND uid=1 GROUP BY groupname

但我得到了那些有价值的团体。

Answer 1

使用Left Outer Join代替旧版Inner Join

SELECT g.groupname,
       Count(c.gid) as Cnt
FROM   groups g
       LEFT OUTER JOIN contact c
                    ON g.gid = c.gid
WHERE  uid = 1
GROUP  BY g.groupname

Answer 2

您可以使用相关的子查询：

SELECT g.gid, g.groupname, g.TYPE, 
       (SELECT COUNT(*) 
        FROM contacts c
        WHERE c.gid = g.gid) AS cnt
FROM groupname g
WHERE  uid=1

Answer 3

SELECT g.gid,g.groupname,g.TYPE,g.DATE,COUNT(*)AS cnt,1 
FROM groupname g 
LEFT JOIN contacts c on c.group_name = g.group_name 
WHERE g.gid=c.gid AND uid=1 GROUP BY groupname

如何在mysql中从另一个表中查找不存在的数据

3 个答案: