Question

I need to segment clients by the first date that their count of orders reached a specific value.

for example, we have a table of orders

ID, ClientID, Date

1,  1, 1/1/2011
2,  1  2/1/2011
3,  1  3/1/2011
4,  1  4/1/2011

I would like to get all the clients who has at least 3 orders and get the Date field when they reached 3 orders.
on my example, the query will show:

ClientID: 1, Date: 3/1/2011

(cause on the 3/1/2011, the client reached 3 orders)

How can I do it?

Thanks

Update:

I’m looking at your solutions (thank you very much!) but I need to do the same thing also with SUM (lets say the table above has a field called amount and I would like to segment the clients and get the first date that the client orders reached to $100)
I though that the solutions will help me understand the logic and easily convert the count to sum and make a new query but I got little bit lost here.. any ideas?

Answer 1

This would select all the clients with 3 orders with the date of 3rd order.

--DECLARE @clientID INT
--SET @clientID = 1
DECLARE @count INT
SET @count = 3      -- Set the count to this variable.

SELECT  ClientID, [Date]
FROM    [yourtable] a
WHERE   COALESCE(    (SELECT COUNT(*) 
                      FROM [yourtable] b 
                      WHERE b.[Date] < a.[Date] and b.ClientID = a.ClientID ),0) = @count - 1
ORDER BY ClientID

To select single client with date of 3rd order where could be modified with additional condition for that.

AND a.ClientID = @clientID

UPDATE based on question update – This query with little modification from count woud give you the running sum selection. There are few more techniques listed here for running sum.

DECLARE @sum INT
SET @count = 100      -- Set the amount to this variable.

SELECT  ClientID, [Date]
FROM    [yourtable] a
WHERE   ( a.Amount + COALESCE(    (SELECT SUM(b.Amount) 
                      FROM [yourtable] b 
                      WHERE b.[Date] < a.[Date] and b.ClientID = a.ClientID ),0) ) = @sum
ORDER BY ClientID