Column is invalid in select

Question

How I will properly do this:

Customer

CusID, CusLname, CusFname, CusMname, CusAddress, CusEmailAdd

Order

OrderID,  Ord         


        

Answer 1

Why do I need to add those columns in the Group By Clause?

Because you are saying you need to group the results and give one result per ID. This leaves you with two choices.

1) Getting the count with all details at the customer level, which is what you would want in your example.

Select  Count(OrderID), o.CusID, CusLname, CusFname, CusMname, CusAddress, CusEmailAdd
From Customer c 
Inner join
Order o
On c.CusID = o.CusID
Group By o.CusID, CusLname, CusFname, CusMname, CusAddress, CusEmailAdd;

2) Or in case the other columns are some numbers (say salaries in a department) or so, every column not in the group by should have an aggregate function.

select dept_id, sum(salary), max(salary), min(salary)
from dept
group by dept_id;

When you want one row per your grouped column, you need to specify how to aggregate the other columns.