是否可以在没有连接的情况下按计数（不同）进行分组？

Question

Table 1 : (Company)

ID Name
1  A
2  B
3  C

每个公司 (pk = ID) 可以拥有一名或多名员工。

Table 2 :  (Employee)  (CompanyID referencing ID)

CompanyID EmpID Name
1         1     Joe
1         2     Doe
1         3     Boe
2         4     Lou
3         5     Su  
3         6     Ram

查询：

select CompanyID, count(*) from Employee group by CompanyID having count(*) > 1; # Lists companies and their counts.

CompanyID count(*)
1         3  
3         2

对于此查询，我只想要一个包含不同 CompanyID 计数的结果。因此，本例中为“2”[公司 A 和 C]。

简而言之，我正在寻找拥有 2 名或更多员工的公司数量。

是否有办法在没有临时表或连接的情况下获得结果？我正在使用MySQL。

Answer 1

是：

select count(*) from
(select CompanyID from Employee group by CompanyID having count(*) > 1) v

或对于范围：

select count(*) from
(select CompanyID from Employee group by CompanyID 
 having count(*) >= 5 and count(*) < 10) v

Answer 2

是的，可以使用子查询：

SELECT COUNT(*)
FROM
  ( SELECT 1
    FROM Employee 
    GROUP BY CompanyID 
    HAVING COUNT(*) > 1
  ) AS grp

或者：

SELECT COUNT(DISTINCT CompanyID) 
FROM Employee AS e
WHERE EXISTS
      ( SELECT *
        FROM Employee AS e2
        WHERE e2.CompanyID = e.CompanyId
          AND e2.EmpID <> e.EmpID
      )

或者如果 COUNT(*) 很慢，您可以使用 MIN() 和 MAX() ：

SELECT COUNT(*)
FROM
  ( SELECT 1 
    FROM Employee 
    GROUP BY CompanyID 
    HAVING MAX(EmpID) > MIN(EmpId)
  ) AS grp