SQL嵌套聚合查询 - 如何获得每位员工的客户数量？

Question

以下是我需要回答的提示：

列出每位员工的客户数量。 包括员工姓名和适当标记的客户数量，列出最先顾客户的员工。

以下是我的EMPLOYEES表：

create table EMPLOYEES
    (EmpID    char(4)         unique Not null,
     Ename    varchar(10),
     Job      varchar(9),
     MGR      char(4),
     Hiredate date,
     Salary   decimal(7,2),
     Comm     decimal(7,2),
     DeptNo   char(2)         not null,
         Primary key(EmpID),
         Foreign key(DeptNo) REFERENCES DEPARTMENTS(DeptNo));


insert into EMPLOYEES values (7839,'King','President',null,'17-Nov-11',5000,null,10);
insert into EMPLOYEES values (7698,'Blake','Manager',7839,'01-May-11',2850,null,30);
insert into EMPLOYEES values (7782,'Clark','Manager',7839,'02-Jun-11',2450,null,10);
insert into EMPLOYEES values (7566,'Jones','Manager',7839,'02-Apr-11',2975,null,20);
insert into EMPLOYEES values (7654,'Martin','Salesman',7698,'28-Feb-12',1250,1400,30);
insert into EMPLOYEES values (7499,'Allen','Salesman',7698,'20-Feb-11',1600,300,30);
insert into EMPLOYEES values (7844,'Turner','Salesman',7698,'08-Sep-11',1500,0,30);
insert into EMPLOYEES values (7900,'James','Clerk',7698,'22-Feb-12',950,null,30);
insert into EMPLOYEES values (7521,'Ward','Salesman',7698,'22-Feb-12',1250,500,30);
insert into EMPLOYEES values (7902,'Ford','Analyst',7566,'03-Dec-11',3000,null,20);
insert into EMPLOYEES values (7369,'Smith','Clerk',7902,'17-Dec-10',800,null,20);
insert into EMPLOYEES values (7788,'Scott','Analyst',7566,'09-Dec-12',3000,null,20);
insert into EMPLOYEES values (7876,'Adams','Clerk',7788,'12-Jan-10',1100,null,20);
insert into EMPLOYEES values (7934,'Miller','Clerk',7782,'23-Jan-12',1300,null,10);

以下是我的CUSTOMERS表：

create table CUSTOMERS
    (CustID      char(6)       unique Not null,
     Name        varchar(45),
     Address     varchar(40),
     City        varchar(30),
     State       varchar(2),
     Zip         varchar(9),
     AreaCode    char(3),
     Phone       varchar (9),
     RepID       char(4)       not null,
     CreditLimit decimal(9,2),
     Primary key(CustID),
         Foreign key(RepID) References EMPLOYEES(EmpID));


insert into CUSTOMERS values (100,'Jocksports','345 Viewridge','Belmont','CA','96711',415,'598-6609',7844,5000);
insert into CUSTOMERS values (101,'TKB Sport Shop','490 Boli Rd.','Redwood City','CA','94061',415,'368-1223',7521,10000);
insert into CUSTOMERS values (102,'Vollyrite','9722 Hamilton','Burlingame','CA','95133',415,'644-3341',7654,7000);
insert into CUSTOMERS values (103,'Just Tennis','Hillview Mall','Burlingame','CA','97544',415,'677-9312',7521,3000);
insert into CUSTOMERS values (104,'Every Mountain','574 Surry Rd.','Cupertino','CA','93301',408,'996-2323',7499,10000);
insert into CUSTOMERS values (105,'K + T Sports','3476 El Paseo','Santa Clara','CA','91003',408,'376-9966',7844,5000);
insert into CUSTOMERS values (106,'Shape Up','908 Sequoia','Palo Alto','CA','94301',415,'364-9777',7521,6000);
insert into CUSTOMERS values (107,'Womens Sports','Valco Village','Sunnyvale','CA','93301',408,'967-4398',7499,10000);
insert into CUSTOMERS values (108,'North Woods Fitness Supply Center','98 Lone Pine Way','Hibbing','MN','55649',612,'566-9123',7844,8000);

以下是我的查询：

select ename, empId
from EMPLOYEES
where EmpID in
(select count(repid) as NumberOfCustomers
from CUSTOMERS
group by RepID);

为什么我的查询不起作用？

我知道我希望将来自EMPLOYEES的empid与CUSTOMERS中的repID相匹配，然后计算代表ID在CUSTOMERS中出现的次数。 我需要EMPLOYEES表的唯一原因是输出Ename。 我很困惑我需要使用的语法，因为我需要输出CUSTOMERS表中的RepID计数

Answer 1

SELECT  *
FROM    employees e
CROSS APPLY
        (
        SELECT  COUNT(*)
        FROM    customers c
        WHERE   c.repId = e.empId
        ) cc (cnt)
ORDER BY
        cnt DESC

在SQLFiddle上看到这个

与子查询相同：

SELECT  *,
        (
        SELECT  COUNT(*)
        FROM    customers c
        WHERE   c.repId = e.empId
        ) cnt
FROM    employees e
ORDER BY
        cnt DESC

Answer 2

如果您使用的是SQL Server 2005及更高版本，也可以使用Common Table Expression 。

;WITH CTE
AS
(
   SELECT RepID, COUNT(*) AS CNT
   FROM CUSTOMERS 
   GROUP BY REPID  
)

SELECT E.Ename, E.EmpID, ISNULL(C.CNT, 0) 
FROM EMPLOYEES E
LEFT OUTER JOIN CTE C ON C.RepID = E.EmpID

DEMO

Answer 3

您的内部查询只有一个count(repid) ，它是一个计数，而不是您的外部查询where EmpID in (xxx)所需的实际EmployeeID。 不确定为什么你想要在你的内部查询中计数，因为你将在最终结果中丢失它，但这应该有效：

 select ename, empId
 from EMPLOYEES
 where EmpID in (
    select repID
    from CUSTOMERS);

Answer 4

那这个呢？ 还是我误解了

 SELECT ename, count(*) as NumCustomers 
FROM Customers INNER JOIN 
 Employees ON customers.repid = employees.empid GROUP BY repid,ename 
 ORDER BY numCustomers DESC