Summing the values from the 2nd table based on ID of the 1st table

Question

I have done this query before, but for some reason I always have to dig the answer up. Can someone explain the solution for me so I can finally 'get it'! (thanks!)

Table#1 Employees (employeeID, username)
Table#2 Sales (saleID, employeeID, amount)

Question: List all the employees, along with the total # (count) of sales they have.

Answer 1

You'd want to select all Employees, and calculate their count of sales for each. Because you'd want all employees in the list, you'd select from the Employees table and either left join to the sales table, or do a subquery to the sales table. Doing this will give you the employees with zero sales in the results as well. In the case of the join, you'd have to group by the employee and count the records in the sales table. For the subquery, there is no group by because your base query will return just 1 row per employee.

select   Employees.EmployeeID, 
         Employees.UserName, 
         CountOfSales = COUNT(SaleID)
from     Employees LEFT JOIN 
         Sales ON Employees.EmployeeID = Sales.EmployeeID
group by Employees.EmployeeID, 
         Employees.UserName
/*
EmployeeID  UserName   CountOfSales
----------- ---------- ------------
2           bill       1
3           larry      0
1           scott      2
Warning: Null value is eliminated by an aggregate or other SET operation.
*/

-- OR --

select   E.*, 
         CountOfSales = (select count(*) 
                         from   sales 
                         where  EmployeeID = E.EmployeeID)
from     Employees E
/*
employeeID  username   CountOfSales
----------- ---------- ------------
1           scott      2
2           bill       1
3           larry      0
*/

Results from each query are based on the sample data below...

create table Employees (employeeID int , username varchar(10)) 
create table Sales (saleID int , employeeID int , amount smallmoney)
go
insert Employees values (1, 'steve'), (2, 'bill'), (3, 'larry')
insert Sales values (1, 1, 23), (2,1,33), (3,2,0)
go

Answer 2

select emp.username
      ,isnull((select count(*)
                 from sales
                where sales.employeeid = emp.employeeid),0) as [number of sales]
  from employees emp

Answer 3

You say you want the sum in the title of the question but then say you want the # (count) in the body.

If you want the sum then use the SUM function in SQL.

select Employees.EmployeeID, TotalSales = SUM(amount)
from Employees LEFT JOIN Sales ON Employees.EmployeeID = Sales.EmployeeID

Answer 4

select
   e.employeeID
   , e.username
   , count(s.saleID) as'sales count'
   , sum(s.amount) as 'sales $ total'
from
   employees e
left outer join
   sales s
on
   s.employeeID = e.employeeID
group by 
   e.employeeID
   , e.username

Answer 5

Isn't it this simple, assuming that one sale can only be made by one employee?

Select Employees.username, count(Sales.saleID)
From Employees Left Join Sales on Employees.employeeID = Sales.employeeID
Group by Employees.username