I am rather new in T-SQL and I have to create a view, where the output will be as shown below:
enter image description here
But my sales table doesn't have any data about sales in February and May for customer ABC and no data in January for customer XYZ, but I really want to have 0 for these months. How to do it in T-SQL?
This is great question about a very important topic that, even many experienced developers need to touch up on. Being "relatively new at SQL" I wont just offer a solution, I'll explain the key concepts involved.
The Auxiliary Table Numbers
First lets learn about what a tally table, aka numbers table is all about.
What does this do?
SELECT N = 1 ;
It returns the number 1.
N
-----
1
How about this?
SELECT N = 1 FROM (VALUES(0)) AS e(N);
Same thing:
N
-----
1
What does this return?
SELECT N = 1 FROM (VALUES(0),(0),(0),(0),(0),(0)) AS e(n);
Here I'm leveraging the VALUES table constructer which allows for a list of values to be treated like a view. This returns:
N
-------
1
1
1
1
1
We don't need the ones, we need the rows. This will make more sense in a moment. Now, what does this do?
WITH e(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0)) AS e(n))
SELECT N = 1 FROM e e1;
It returns the same thing, five 1's, but I've wrapped the code into a CTE named e. Think of CTEs as inline unnamed views that you can reference multiple times. Now lets CROSS JOIN e to itself. This returns for 25 dummy rows (5*5).
WITH e(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0)) AS e(n))
SELECT N = 1 FROM e e1, e e2;
Next we leverage ROW_NUMBER() over our set of dummy values.
WITH E1(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0)) AS e(n))
SELECT N = ROW_NUMBER() OVER (ORDER BY(SELECT NULL)) FROM E1, E1 a;
Returns (truncated for brevity):
N
--------------------
1
2
3
...
24
25
Using as an auxiliary numbers table
#OneToTen is a table with random numbers 1 to 10. I need to count how many there are, returning 0 when there aren't any. NOTE MY COMMENTS:
;--== 2. Simple Use Case - Counting all numbers, including missing ones (missing = 0)
DECLARE #OneToTen TABLE (N INT);
INSERT #OneToTen VALUES(1),(2),(2),(2),(4),(8),(8),(10),(10),(10);
WITH E1(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) AS e(n)),
iTally(N) AS (SELECT ROW_NUMBER() OVER (ORDER BY(SELECT NULL)) FROM E1, E1 a)
SELECT
N = i.N,
Wrong = COUNT(*), -- WRONG!!! Don't do THIS, this counts ALL rows returned
Correct = COUNT(t.N) -- Correct, this counts numbers from #OneToTen AKA "t.N"
FROM iTally AS i -- Aux Table of numbers
LEFT JOIN #OneToTen AS t -- Table to evaluate
ON i.N = t.N -- LEFT JOIN #OneToTen numbers to our Aux table of numbers
WHERE i.N <= 10 -- We only need the numbers 1 to 10
GROUP BY i.N; -- Group by with no Sort!!!
This returns:
N Wrong Correct
----- ----------- -----------
1 1 1
2 3 3
3 1 0
4 1 1
5 1 0
6 1 0
7 1 0
8 2 2
9 1 0
10 3 3
Note that I show you the wrong and right way to do this. Note how COUNT(*) is wrong for this, you need COUNT(whatever you are counting).
Auxiliary table of Dates (AKA calendar table)
My we use our numbers table to create a calendar table.
;--== 3. Auxilliary Month/Year Calendar Table
DECLARE #Start DATE = '20191001',
#End DATE = '20200301';
WITH E1(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) AS e(n)),
iTally(N) AS (SELECT ROW_NUMBER() OVER (ORDER BY(SELECT NULL)) FROM E1, E1 a)
SELECT TOP(DATEDIFF(MONTH,#Start,#End)+1)
TheDate = f.Dt,
TheYear = YEAR(f.Dt),
TheMonth = MONTH(f.Dt),
TheWeekday = DATEPART(WEEKDAY,f.Dt),
DayOfTheYear = DATEPART(DAYOFYEAR,f.Dt),
LastDayOfMonth = EOMONTH(f.Dt)
FROM iTally AS i
CROSS APPLY (VALUES(DATEADD(MONTH, i.N-1, #Start))) AS f(Dt)
This returns:
TheDate TheYear TheMonth TheWeekday DayOfTheYear LastDayOfMonth
---------- ----------- ----------- ----------- ------------ --------------
2019-10-01 2019 10 3 274 2019-10-31
2019-11-01 2019 11 6 305 2019-11-30
2019-12-01 2019 12 1 335 2019-12-31
2020-01-01 2020 1 4 1 2020-01-31
2020-02-01 2020 2 7 32 2020-02-29
2020-03-01 2020 3 1 61 2020-03-31
You will only need the YEAR and MONTH.
The Auxiliary Customer table
Because you are performing aggregations (SUM,COUNT,etc.) against multiple customers we will also need an Auxiliary table of customers, more commonly known as a lookup or dimension.
SAMPLE DATA:
;--== Sample Data
DECLARE #sale TABLE
(
Customer VARCHAR(10),
SaleYear INT,
SaleMonth TINYINT,
SaleAmt DECIMAL(19,2),
INDEX idx_cust(Customer)
);
INSERT #sale
VALUES('ABC',2019,12,410),('ABC',2020,1,668),('ABC',2020,1,50), ('ABC',2020,3,250),
('CDF',2019,10,200),('CDF',2019,11,198),('CDF',2020,1,333),('CDF',2020,2,5000),
('CDF',2020,2,325),('CDF',2020,3,1105),('FRED',2018,11,1105);
Distinct list of customers for an "Auxilliary Table of Customers"
SELECT DISTINCT s.Customer FROM #sale AS s;
For my sample data we get:
Customer
----------
ABC
CDF
FRED
Putting it all together
Here I'm going to:
Create a numbers table
Use my numbers table to create a calendar table
Create an auxiliary Customer table from #sale
CROSS JOIN (combine) both tables for a "junk dimension"
LEFT JOIN our sales data to our calendar/customer auxiliary tables/junk dimension
Group by the auxiliary table values
SOLUTION:
;--==== SAMPLE DATA
DECLARE #sale TABLE
(
Customer VARCHAR(10),
SaleYear INT,
SaleMonth TINYINT,
SaleAmt DECIMAL(19,2),
INDEX idx_cust(Customer)
);
INSERT #sale
VALUES('ABC',2019,12,410),('ABC',2020,1,668),('ABC',2020,1,50), ('ABC',2020,3,250),
('CDF',2019,10,200),('CDF',2019,11,198),('CDF',2020,1,333),('CDF',2020,2,5000),
('CDF',2020,2,325),('CDF',2020,3,1105),('FRED',2018,11,1105);
;--==== START/END DATEs
DECLARE #Start DATE = '20191001',
#End DATE = '20200301';
;--==== FINAL SOLUTION
WITH -- 6.1. Auxilliary Table of numbers:
E1(N) AS (SELECT 1 FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) AS e(n)),
iTally(N) AS (SELECT ROW_NUMBER() OVER (ORDER BY(SELECT NULL)) FROM E1, E1 a),
-- 6.2. Use numbers table to create an "Auxilliary Date Table" (Calendar Table):
MonthYear(SaleYear,SaleMonth) AS
(
SELECT TOP(DATEDIFF(MONTH,#Start,#End)+1) YEAR(f.Dt), MONTH(f.Dt)
FROM iTally AS i
CROSS APPLY (VALUES(DATEADD(MONTH, i.N-1, #Start))) AS f(Dt)
)
SELECT
Customer = cust.Customer,
MonthYear = CONCAT(cal.SaleYear,'-',cal.SaleMonth),
Sales = ISNULL(SUM(s.SaleAmt),0)
-- Auxilliary Table of Customers
FROM (SELECT DISTINCT s.Customer FROM #sale AS s) AS cust -- 6.3. Aux Customer Table
CROSS JOIN MonthYear AS cal -- 6.4. Cross join to create Calendar/Customer Junk Dimension
LEFT JOIN #sale AS s -- 6.5. Join #sale to Junk Dimension on Year,Month and Customer
ON s.SaleYear = cal.SaleYear
AND s.SaleMonth = cal.SaleMonth
AND s.Customer = cust.Customer
GROUP BY cust.Customer, cal.SaleYear, cal.SaleMonth -- 6.6. Group by Junk Dim values
ORDER BY cust.Customer, cal.SaleYear, cal.SaleMonth; -- Order by not required
RESULTS:
Customer MonthYear Sales
---------- ------------ ------------
ABC 2019-10 0.00
ABC 2019-11 0.00
ABC 2019-12 410.00
ABC 2020-1 718.00
ABC 2020-2 0.00
ABC 2020-3 250.00
CDF 2019-10 200.00
CDF 2019-11 198.00
CDF 2019-12 0.00
CDF 2020-1 333.00
CDF 2020-2 5325.00
CDF 2020-3 1105.00
FRED 2019-10 0.00
FRED 2019-11 0.00
FRED 2019-12 0.00
FRED 2020-1 0.00
FRED 2020-2 0.00
FRED 2020-3 0.00
I have a table of events, called tbl_events that looks something like this:
PersonID Date
1 30/03/2015
1 22/04/2015
1 30/06/2015
2 18/07/2016
2 09/12/2016
2 28/04/2017
3 01/10/2014
3 28/11/2016
3 28/11/2016
3 16/01/2017
4 13/04/2017
4 09/05/2017
I want to be able to group these events up by the start date of each 'sequence', with a sequence being defined as a run of events from the first identified to the last identified for each PersonID. The last event in a sequence is defined as the event where thereafter there are no subsequent events for that PersonID for a year.
The result of this I would expect to look like is below:
PersonID FirstDate Sequence Events
1 30/03/2015 1 3
2 18/07/2016 1 3
3 01/10/2014 1 1
3 28/11/2016 2 3
4 13/04/2017 1 2
I am able to identify the sequences in Excel and pivot the data, but I need to be able to do this in SQL.
Here is the formula I have used in Excel to generate the sequence number (I am populating cell C3, with column A being PersonID and B being Date):
=+IF(A2<>A3,1,IF((B3-B2)<365,C2,C2+1))
I have joined the table back on itself using ROW_NUMBER to get the difference between the Date and the previous event date for that ID, but I'm not really sure where to go from there.
Any help is much appreciated.
My solution is based on the sample data you've provided along with your excel formula.
-- easily consumable sample data
DECLARE #tbl_events TABLE (PersonId int, [date] date)
INSERT #tbl_events VALUES
(1,'20150330'),(1,'20150422'),(1,'20150630'),(2,'20160718'),(2,'20161209'),(2,'20170428'),
(3,'20141001'),(3,'20161128'),(3,'20161128'),(3,'20170116'),(4,'20170413'),(4,'20170509');
-- Solution
WITH groupings AS
(
SELECT
PersonId,
FirstDate = MIN([date]) OVER (PARTITION BY personId ORDER BY [date]),
NextDate = LAG([date],1,[date]) OVER (PARTITION BY personId ORDER BY [date]),
[date],
grouper =
DATEDIFF(DAY, MIN([date]) OVER (PARTITION BY personId ORDER BY [date]), [date]) / 365
FROM #tbl_events
),
Prep AS
(
SELECT
PersonId,
firstDate = IIF(grouper = 0, FirstDate, IIF(FirstDate = NextDate, [date],NextDate))
FROM groupings
)
SELECT
PersonId,
FirstDate,
[Sequence] = ROW_NUMBER() OVER (PARTITION BY personId ORDER BY FirstDate),
[Events] = COUNT(*)
FROM prep
GROUP BY personId, FirstDate;
Results
PersonId FirstDate Sequence Events
----------- ---------- -------------------- -----------
1 2015-03-30 1 3
2 2016-07-18 1 3
3 2014-10-01 1 1
3 2016-11-28 2 3
4 2017-04-13 1 2
First note all years have 365 days, nonetheless, I'm using 365 to emulate your excel logic; this would need to be updated to account for leap years. Next, like your excel formula - this will only be correct when there are two sequences;
it would not work when, say personId has a date of jan 1 2015, then jan 10 2016, then feb 1 2017.Let us know if we need logic to accommodate for the aforementioned scenarios.
Lastly this solution uses LAG which requires SQL Server 2012+, if you're working with an earlier version of SQL the query will have to be updated accordingly.
I have two table "book" and "authorCollection". Because a book may have multi-authors, I hope to get the average number of authors in table "book" which published after year 2000(inclusive).
For example:
Table Book:
key year
1 2000
2 2001
3 2002
4 1999
Table authorCollection:
key author
1 Tom
1 John
1 Alex
1 Mary
2 Alex
3 Tony
4 Mary
The result should be (4 + 1 + 1) / 3 = 2;(key 4 publish before year 2000).
I write the following query statement, but not right, I need to get the number of result in subquery, but cannot give it a label "b", How can i solve this problem? And get the average number of author? I still confused about "COUNT(*) as count" meaning....Thanks.
SELECT COUNT(*) as count, b.COUNT(*) AS total
FROM A
WHERE key IN (SELECT key
FROM Book
WHERE year >= 2000
) b
GROUP BY key;
First, count number of authors for a key in a subquery. Next, aggregate needed values:
select avg(coalesce(ct, 0))
from book b
left join (
select key, count(*) ct
from authorcollection
group by 1
) a
using (key)
where year >= 2000;
A sample as well as handling 'divide by zero' error:
select case when count(distinct book.key)=0
then null
else count(authorCollection.key is not null)/count(distinct book.key)
end as avg_after_2000
from book
left join authorCollection on(book.key=authorCollection.key)
where book.year >= 2000