SQL - Query records within an undefined interval

Question

I need to get the following data from Orders table in Northwind database using SQL (SQL Server to be exact):

• Find all customers ( CustomerID ) who placed at least 3 orders within the interval of 6 months ( OrderDate ).

So for the following data the query should return 1

==========================
| CustomerID | OrderDate |
==========================
|      1     | 2000-1-1  |
--------------------------
|      1     | 2000-1-5  |
--------------------------
|      1     | 2000-4-30 |
--------------------------
|      2     | 2000-1-1  |
--------------------------
|      2     | 2000-5-30 |
--------------------------

Answer 1

SELECT N1.CustomerID, COUNT(N2.OrderDate)  FROM Northwind N1
INNER JOIN Northwind N2
ON N1.CustomerID=N2.CustomerID
AND N1.OrderDate<=Dateadd(MONTH, 6, N2.OrderDate)
GROUP BY N1.CustomerID
HAVING COUNT(N2.OrderDate)>=3

Answer 2

请尝试以下操作：

SELECT OrderDate, CustomerID FROM orders GROUP BY OrderDate HAVING   COUNT(CustomerID) >= 3 ORDER BY COUNT(CustomerID) DESC