SQL query with duplicate records

Question

I am trying to write a query in SQL server to find out if there are any multiple rows for each customer by customerID. Please let me know.

Here is the table structur

Answer 1

You can use a GROUP BY query to achieve this:

select CustomerID, count(*) as NumDuplicates
from Customer
group by CustomerID
having count(*) > 1

Answer 2

To see how many of each customer you have:

SELECT COUNT(*), CustName, CustomerID
from Customer
Group by CustName, CustomerID

You can use a having clause to limit to just duplicates:

SELECT COUNT(*), CustName, CustomerID
from Customer
Group by CustName, CustomerID
having count(*) > 1

UPDATE

To get those with successful orders:

select count(*), CustName, CustomerID
from(
  SELECT CustName, CustomerID
  from Customer, orders
  where customer.orderID = orders.orderID
  and orders.success = 1) subquery
group by subquery.CustName, subquery.CustomerID
having count(*) > 1; 

Answer 3

select CustomerID, count(1)
  from Customer
 group by CustomerID
having count(1) > 1