Join SQL Statements optimization - sql

Question

I have 2 Tables:
Customer
ID
Customer_ID
Name
Sir_Name
Phone
Email
and
Table Invoice
Manager_Name
Manaer_First_Name
Customer_ID1
Customer_ID2
Customer_ID3
There is only one Customer.Customer_ID for each Customer or a Customer has no Customer_ID
In Invoice.Customer_ID1 i have the same Customer_ID.Customer_ID several times.
I Like to get all Records in Customer Table Join Invoice Table - check if the Customer_ID = Customer_ID1 if not check in Customer_ID = Customer_ID2 Or Customer_ID = Customer_ID2
If customer_ID is found in one of rows stop the search.

Answer 1

Probably the best way to write the query is:
select . . .
from customer c join
invoice i
on c.customer_id = coalesce(i.customer_id1, i.customer_id2, i.customer_id3);
This should be able to take advantage of an index on customer(customer_id). If this is not efficient, then another alternative is left join:
select . . ., coalesce(c1.col1, c2.col1, c3.col1) as col1, . . .
from invoice i left join
customer c1
on c1.customer_id = i.customer_id1 left join
customer c2
on c2.customer_id = i.customer_id2 left join
customer c3
on c3.customer_id = i.customer_id3;
The left join can take advantage of an index on customer(customer_id). You need to use coalesce() in the select to choose the field from the right table.

Answer 2

select
*
from [Table Invoice] A
JOIN [Customer] B
ON B.Customer_ID = A.Customer_ID1 OR (B.Customer_ID <> A.Customer_ID1 AND B.Customer_ID = A.Customer_ID2) OR (B.Customer_ID = A.Customer_ID3 AND B.Customer_ID <> A.Customer_ID2 AND B.Customer_ID <> A.Customer_ID1)
this would return you all the Invoices for all of the Customers. In case you need Invoices just for one customer - add
WHERE B.Customer_ID = #YourCustomerID
statement. If you need only one, first invoice, add 'TOP 1' to select statement:
SELECT TOP 1

Answer 3

Could a inner join on or clause
select Customer.*, Invocie.*
from Customer
inner join Invoice on ( Customer.Customer_ID = Invoce.Customer_ID1
OR Customer.Customer_ID = Invoce.Customer_ID2
OR Customer.Customer_ID = Invoce.Customer_ID3)

Answer 4

This is how I understand your request: You want all customers that have at least one entry in the invoice table. But per customer you want the "best" invoice record only; with ID1 match better than ID2 match and ID2 match better than ID3 match.
So join the tables to get all matches and then rank your matches with row_number giving the best matching record #1. Then only keep those rows ranked #1.
select *
from
(
select
c.*,
i.*,
row_number() over
(
partition by c.customer_id order by
case c.customer_id
when i.customer_id1 then 1
when i.customer_id2 then 2
when i.customer_id3 then 3
end
) as rn
from customer c
join invoice i on c.customer_id in (i.customer_id1, i.customer_id2, i.customer_id3)
)
where rn = 1;