SQL query with duplicate records

I am trying to write a query in SQL server to find out if there are any multiple rows for each customer by 开发者_如何学GocustomerID. Please let me know.

Here is the table structure

Customer table
-----------------------------------------
orderID          CustName      CustomerID
---------------------------------------
100               test           123456    
101               test           123456

Orders table
------------------------------------
pID               OrderID
-----------------------------------
1                 100        
2                 101

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You can use a GROUP BY query to achieve this:

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

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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; 

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