With SqlServer, where * not exists - sql

Question

I got two tables, Orders with two columns as orderid and customerid, and Customers with two columns as customerid and location.
What I'd like to do is find all the customerid in the table Customers, which are not in Orders. For example, Customers.customerid = {A, B, C, D}, Orders.customerid = {A, B, C}, guess what I need to do is just get the ones from Customers but not exists in Orders. For achieving that, I put,
select customerid from Customers where customerid not exists (select customerid from Orders)
But it returns nothing.. My logic is quite simple like, first got all customerid in table Orders, then get the ones which doesn't exisit in the customerIds from table Orders. I can't see why this is wrong..
I tried this later, and it works. May anyone can help me pls?
select customerid from Customers as c where customerid not exists(select customerid from orders as o where c.customerid = o.customerid)
Why do I have to add c.customerid = o.customerid?

Answer 1

Why do I have to add c.customerid = o.customerid?
Because just because you're using the same name for two columns in your database, that doesn't mean that any specific relationship is enforced or assumed between them.
You need to add the c.customerid = o.customerid to specify that you're interested in the specific condition that these two columns are equal.
But any other correlation condition is also allowed by the language. E.g. you could write a query:
select customerid from Customers as c where not exists(
select customerid from Customers as c2 where c2.customerid < c.customerid)
Which would find you the "first" customer, if considering the customers sorted by their customerid values (not that this is the best way of writing this query, it's just a demonstration of the flexibility)
Your first query was, in effect "give me all rows from the Customer table, provided that no rows exist in the Order table" - which is also a perfectly valid thing to ask for, but wasn't what you intended - you intended to perform some form of correlation, which is what you did in your second query.

Answer 2

May be you need:
select customerid from Customers where customerid not in (select customerid from Orders)

Answer 3

Try below query :
SELECT customerid from Customers C WHERE NOT EXISTS
(
SELECT 1 FROM orders O WHERE C.customerid = O.customerid
)

Answer 4

what you need is
select c.customerid from customer c inner join order o on c.customerid = o.customerid where c.customerid not in (select od.customerid from order od)
you can't access data of 2 tables without joining them.

Answer 5

The syntax is a bit off. You mean to write it like this:
SELECT C.CustomerID
FROM Customers C
WHERE NOT EXISTS
(
SELECT O.CustomerID
FROM Orders O
WHERE O.CustomerID = C.CustomerID
)
;
You could also do this with NOT IN, as such:
SELECT customerid
FROM Customers
WHERE customerid NOT IN
(
SELECT customerid
FROM Orders
)
;
The two are semantically equivalent, for the most part.
Some people will probably tell you that you could do the same thing with a LEFT JOIN / IS NULL construct, but you can look at this article to see why that is a poorer choice in many circumstances.

Answer 6

#Damien_The_Unbeliever gave correct explanation and u need to try like this
with some data i created for 2 tables
CREATE TABLE #Orders
(orderid varchar(10), customerid varchar(10))
insert into #Orders values
('venkat','a'),
('raj','b'),
('mahes','c')
CREATE TABLE #Customers
(customerid varchar(10), [location] varchar(10))
insert into #Customers values
('a','and'),
('b','bar'),
('c','board'),
('D','board1')
SELECT cu.customerid from #Customers CU WHERE NOT EXISTS
(
SELECT 1 FROM #orders b WHERE Cu.customerid = b.customerid
)
output
customerid
D