Improve the query by removing the sub query - sql-server

Question

I have a Customer table
+--------+---------+
| Id | Name |
+--------+---------+
| 1 | A |
| 2 | b |
| 3 | c |
| 4 | d |
| 5 | 3 |
| 6 | f |
| 7 | g |
+--------+---------+
and an order table
+-----+------+--------------------------+
| ID | C_Id | OrderDate |
+-----+------+--------------------------+
| 1 | 1 | 2017-05-12 00:00:00.000 |
| 2 | 2 | 2017-12-12 00:00:00.000 |
| 3 | 3 | 2017-11-12 00:00:00.000 |
| 4 | 4 | 2017-12-12 00:00:00.000 |
| 5 | 1 | 2017-12-12 00:00:00.000 |
| 6 | 2 | 2017-12-12 00:00:00.000 |
| 7 | 3 | 2017-12-12 00:00:00.000 |
| 8 | 4 | 2017-11-12 00:00:00.000 |
| 9 | 2 | 2017-06-12 00:00:00.000 |
| 10 | 3 | 2017-07-12 00:00:00.000 |
+-----+------+--------------------------+
I need the result of the customers who did not buy in last month.
That is from the order table Customer 3 and 4 have purchased in last month(November). The result should not include customer 3 and 4 even they had purchase in earlier months.
I have this query which returns the result perfectly.
SELECT C_ID , MONTH(OrderDate) from [Order]
WHERE MONTH(OrderDate) <> MONTH(GETDATE()) - 1
AND C_ID NOT IN (
SELECT C_ID FROM [Order]
WHERE MONTH(OrderDate) = MONTH(GETDATE()) - 1)
Can anyone help me to write this query without using subquery
EDIT: For more clarity, I need to exclude the customers from the result(get all orders for current year) if they had any purchase in November, also I need results for this year alone.

Answer 1

I think you need to use another way for checking earlier purchases because with MONTH(OrderDate) <> MONTH(GETDATE()) - 1 you have a problem if purchases are in different years.
You need to expand your condition. For example
(
(MONTH(OrderDate)<MONTH(DATEADD(MONTH,-1,GETDATE())) AND YEAR(OrderDate)=YEAR(DATEADD(MONTH,-1,GETDATE())))
OR YEAR(OrderDate)<YEAR(DATEADD(MONTH,-1,GETDATE()))
)
Or you can use the EOMONTH function (from SQL Server 2012) for it. I think this variant will be more useful.
SELECT
C_ID,
MONTH(OrderDate)
FROM [Order]
WHERE EOMONTH(OrderDate)<EOMONTH(DATEADD(MONTH,-1,GETDATE())) -- check for month and year
AND C_ID NOT IN (
SELECT C_ID FROM [Order]
WHERE EOMONTH(OrderDate)=EOMONTH(DATEADD(MONTH,-1,GETDATE())) -- check for month and year
)
I think here is more useful using a variable
DECLARE #lastMonth date=EOMONTH(DATEADD(MONTH,-1,GETDATE()))
SELECT
C_ID,
MONTH(OrderDate)
FROM [Order]
WHERE EOMONTH(OrderDate)<#lastMonth -- check for month and year
AND C_ID NOT IN (
SELECT C_ID FROM [Order]
WHERE EOMONTH(OrderDate)=#lastMonth -- check for month and year
)
A variant without a subquery
SELECT
C_ID,
MIN(OrderDate) FirstOrderDate,
MAX(OrderDate) LastOrderDate
FROM [Order]
WHERE OrderDate<=EOMONTH(DATEADD(MONTH,-1,GETDATE()))
GROUP BY C_ID
HAVING EOMONTH(MAX(OrderDate))<EOMONTH(DATEADD(MONTH,-1,GETDATE()))
Or
DECLARE #lastMonth date=EOMONTH(DATEADD(MONTH,-1,GETDATE()))
SELECT
C_ID,
MIN(OrderDate) FirstOrderDate,
MAX(OrderDate) LastOrderDate
FROM [Order]
WHERE OrderDate<=#lastMonth
GROUP BY C_ID
HAVING EOMONTH(MAX(OrderDate))<#lastMonth
But here I return only MIN(OrderDate) and MAX(OrderDate) but maybe it'll suit you.
I don't think that a variant with a subquery is worse. I think it's more clearly.
DECLARE #lastMonth date=EOMONTH(DATEADD(MONTH,-1,GETDATE()))
SELECT
C_ID,
YEAR(OrderDate) [Year],
MONTH(OrderDate) [Month]
COUNT(ID) OrderCount
FROM [Order]
WHERE EOMONTH(OrderDate)<#lastMonth
--AND YEAR(OrderDate)=YEAR(#lastMonth) -- if you need only orders from this year
AND C_ID IN(
SELECT DISTINCT C_ID
FROM [Order]
WHERE EOMONTH(OrderDate)=#lastMonth
)
GROUP BY C_ID,YEAR(OrderDate),MONTH(OrderDate)

Answer 2

When working with dated information you cannot just use a month number because month 1 comes after month 12 (of last year). So, work with dates, not month numbers.
For this query we need "this month" and "last month" by their dates (not the month numbers) and we can start with this by using getdate()
A useful "trick" here is to calculate the first day of this month which we can do by calculating the number of months from zero datediff(month,0, getdate() ) then adding that number to zero dateadd(month, ..., 0). So once we have the first of this month it is easy to calculate first of last month and first of next month simply by subtracting or adding 1 month.
So, for a solution that will work in any version of SQL Server:
SQL Fiddle
MS SQL Server 2014 Schema Setup:
CREATE TABLE Orders
([ID] int, [C_Id] int, [OrderDate] datetime)
;
INSERT INTO Orders
([ID], [C_Id], [OrderDate])
VALUES
(1, 1, '2017-05-12 00:00:00'),
(2, 2, '2017-12-12 00:00:00'),
(3, 3, '2017-11-12 00:00:00'),
(4, 4, '2017-12-12 00:00:00'),
(5, 1, '2017-12-12 00:00:00'),
(6, 2, '2017-12-12 00:00:00'),
(7, 3, '2017-12-12 00:00:00'),
(8, 4, '2017-11-12 00:00:00'),
(9, 2, '2017-06-12 00:00:00'),
(10, 3, '2017-07-12 00:00:00')
;
CREATE TABLE Customers
([Id] int, [Name] varchar(1))
;
INSERT INTO Customers
([Id], [Name])
VALUES
(1, 'A'),
(2, 'b'),
(3, 'c'),
(4, 'd'),
(5, '3'),
(6, 'f'),
(7, 'g')
;
Query 1:
declare #this_month datetime = dateadd(month, datediff(month,0, getdate() ), 0)
declare #last_month datetime = dateadd(month,-1,#this_month)
select
c.Id
, c.name
, count(case when o.OrderDate >= #last_month and o.OrderDate < #this_month then 1 end) last_month
, count(case when o.OrderDate >= #this_month then 1 end) this_month
from customers c
LEFT join orders o on c.id = o.c_id
and OrderDate >= #last_month
and OrderDate < dateadd(month,1,#this_month)
group by c.Id, c.name
having count(case when o.OrderDate >= #last_month and o.OrderDate < #this_month then 1 end) = 0
and count(case when o.OrderDate >= #this_month then 1 end) > 0
Results:
| Id | name | last_month | this_month |
|----|------|------------|------------|
| 1 | A | 0 | 1 |
| 2 | b | 0 | 2 |
----
declare #this_year datetime = dateadd(year, datediff(year,0, getdate() ), 0)
declare #this_month datetime = dateadd(month, datediff(month,0, getdate() ), 0)
declare #last_month datetime = dateadd(month,-1,#this_month)
select
c.Id
, c.name
, count(case when o.OrderDate >= #last_month and o.OrderDate < #this_month then 1 end) last_month
, count(o.OrderDate) this_year
from customers c
LEFT join orders o on c.id = o.c_id
and OrderDate >= #this_year
and OrderDate < dateadd(year,1,#this_year)
group by c.Id, c.name
having count(case when o.OrderDate >= #last_month and o.OrderDate < #this_month then 1 end) = 0
and count(o.OrderDate) > 0
;