select orders from first time customers - sql

Question

I need help building a SQL query that returns orders from customers who have only ordered once.
The tables and relevant fields are as follows:
Order Customer
------- -----------
orderId customerId
orderDate
customerId
etc.
I'm looking for a result set of Order records where there is only one occurence of the customer id. For the following data set...
[orderId] [customerId] [orderDate] [etc.]
---------- ------------ ------------ ------------
o1 c1 1/1/14 foo
o2 c2 1/1/14 baz
o3 c3 1/3/14 bar
o4 c2 1/3/14 wibble
I would like the results to be
[orderId] [orderDate] [etc.]
--------- ----------- ------
o1 1/1/14 foo
o3 1/3/14 bar
Orders o2 and o4 are ommitted because c2 has ordered twice.
Any help would be greatly appreciated.
Sorry, didn't put my failed attempt. This is what I tried...
SELECT customerId,
orderId,
orderDate,
Count(*)
FROM Orders
GROUP BY orderId,
orderDate,
customerID
HAVING Count(*) = 1
ORDER BY orderId
It appears to return all the orders.

Answer 1

Try the following (assuming SQL Server 2005+):
;WITH CTE AS
(
SELECT *,
N = COUNT(*) OVER(PARTITION BY customerId)
FROM Orders
)
SELECT *
FROM CTE
WHERE N = 1
Since sometimes a pedestrian approach is preferred over complex CTEs, you can use a derived table if you want (but since it's using the OVER clause, you'll still need SQL Server 2005+):
SELECT *
FROM ( SELECT *,
N = COUNT(*) OVER(PARTITION BY customerId)
FROM Orders) T
WHERE N = 1
Alternatively (if for example you are in an older than 2005 version of SQL-Server), you can use the GROUP BY / HAVING COUNT(*)=1 method to find customers with only 1 order and then join back to the Orders table (no need for aggregate functions in all the columns):
SELECT o.*
FROM Orders o
JOIN
( SELECT customerId
FROM Orders
GROUP BY customerId
HAVING COUNT(*) = 1
) c
ON c.customerId = o.customerId ;
or use NOT EXISTS (no COUNT() needed and it works even in MySQL):
SELECT o.*
FROM Orders o
WHERE NOT EXISTS
( SELECT 1
FROM Orders c
WHERE c.customerId = o.customerId
AND c.orderId <> o.orderId
) ;

Answer 2

This will list all first-time customers in your ORDERS table.
SELECT [customerID],
MIN([orderId]) AS [orderId],
MIN([orderDate]) AS [orderDate],
MIN([etc.]) AS [etc.]
FROM [Orders]
GROUP BY [customerID]
HAVING Count(*) = 1
ORDER BY [customerID]
In order to bring back all the additional columns you would need to wrap them in an aggregate such as MIN/MAX.
It is arbitrary which to use as there will only be one row per group anyway. This does assume that all columns in the table are of datatypes valid for such aggregation however (examples of datatypes that aren't are BIT, or XML)