Joining 3 tables on sql and checking conditions - mysql

Question

SelectClassicModels.Orders.CustomerNumber,
ClassicModels.CUSTOMERS.CUSTOMERNAME,
ClassicModels.Employees.LASTNAME,
ClassicModels.Employees.firstNAME,
ClassicModels.employees.EmployeeNumber
from ClassicModels.Orders
join
ClassicModels.Customers
on ClassicModels.Orders.CustomerNumber = ClassicModels.Customers.CUSTOMERNUMBER
join
ClassicModels.EMPLOYEES
on ClassicModels.Employees.EMPLOYEENUMBER = ClassicModels.CUSTOMERS.SalesRepEmployeeNumber
Those three tables when joined work just fine, but when I try and add these modifiers, they don't work
group by ClassicModels.Orders.CustomerNumber
having count(ClassicModels.Orders.CustomerNumber) < 4

Answer 1

First off, welcome to StackOverflow!
I've re-formatted your SQL using my favorite tool, which I can provide a link to if you're interested. I've also added aliases to help make it more "readable". (The aliases are the lowercase bits after the table name in the FROM and JOIN clauses.)
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
Now that we've got that done. Let's add your GROUP BY and HAVING clauses.
GROUP BY clauses have to have ALL of the columns used in the SELECT clause in them. (I'm not sure why. I haven't looked it up, but I just know that's how it works. :) )
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
GROUP BY orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
Now that should work. Then you just have to add your HAVING clause in there.
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
GROUP BY orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
HAVING COUNT(orders.CustomerNumber) < 4
I was also looking over your query, and you might get faster (and more efficient results) by using a query like this:
WITH CUSTOMERSWITHLESSTHANFOURORDERS
AS
(
SELECT CUSTOMERNUMBER
FROM CLASSICMODELS.ORDERS
GROUP BY CUSTOMERNUMBER
HAVING COUNT(CUSTOMERNUMBER) < 4
)
SELECT O.CUSTOMERNUMBER,
C.CUSTOMERNAME,
E.LASTNAME,
E.FIRSTNAME,
E.EMPLOYEENUMBER
FROM CUSTOMERSWITHLESSTHANFOURORDERS O
JOIN CLASSICMODELS.CUSTOMERS C
ON O.CUSTOMERNUMBER = C.CUSTOMERNUMBER
JOIN CLASSICMODELS.EMPLOYEES E
ON E.EMPLOYEENUMBER = C.SALESREPEMPLOYEENUMBER;
It uses what is called "common table expression" and basically just isolates a portion of the query. It could be more efficient because it's going to try and group on less data, so it could be faster. Be wary, because there's lots of "could"'s in there because I don't know how various different things are set up in your MySQL database.
Good luck!