SQL Server 2014 - Return Team name based on most recent date (somewhat dynamically) - sql

Question

My title is misleading because I don't know how to sum it up better than that :)
I have a table that keeps a history of changes made to users and what teams they belong to. It starts with their initial team and date, then adds an entry via a trigger when we change their teams in the UserList table.
Our business, like many, loves month to month data. I don't want to have entries for every single month if they don't change teams. Ill get to why that's a problem.
Here is an example of the data in the TeamHistory Table
UserID|CurrentTeam|ChangeDate
User1-|Team1------|01-01-2016
User1-|Team2------|03-01-2016
When I run a view or query that rolls the data up by person and media type (I can have 4 entries for a single person in a single month - voice, fax, email and voicemail) I then need to add the team that they were working on for that month.
Using that above example, if I ran the data for all of last year, I would expect Jan-May to display Team1. Then from June to Dec, Team 2. The problem is if I join the date field in my view/query with this table and use an = sign, then I only get data for 1-1 and 6-1, clearly because I only have those values in the table to match against. If I tell it to do < or <=, I start encountering duplicates as its just not specific enough.
If we need an example query, I can try to work something up that's not one of these massive views.
So lets assume this is my data:
Userid| Month |Media|Calls
User1-|-01/01/2016|Voice|200
User1-|-01/01/2016|Email|100
User1-|-02/01/2016|Voice|250
User1-|-02/01/2016|Email|120
User1-|-03/01/2016|Voice|250
User1-|-03/01/2016|Email|120
And the TeamHistory table has 2 entries, the team they started on for 1/1/2016 and then they switched for 3/1/2016. How do I join the two data sets, using the date and userid as my variables, to pull in the corresponding Team? Especially when I wont have an actual entry for 2/1/2016?
Id want my final dataset to look like this:
Userid|Team | Month |Media|Calls
User1-|Team1|-01/01/2016|Voice|200
User1-|Team1|-01/01/2016|Email|100
User1-|Team1|-02/01/2016|Voice|250
User1-|Team1|-02/01/2016|Email|120
User1-|Team2|-03/01/2016|Voice|250
User1-|Team2|-03/01/2016|Email|120

Answer 1

Since you're using SQL Server (2012 and newer) you can use the LEAD() function to identify an end date for a given range:
;with cte aS (SELECT 'User1' as UserID, 'Team1' AS CurrentTeam, CAST('2016-01-01' AS DATE) as ChangeDate
UNION SELECT 'User1' as UserID, 'Team2' AS CurrentTeam, CAST('2016-06-01' AS DATE) as ChangeDate
UNION SELECT 'User1' as UserID, 'Team1' AS CurrentTeam, CAST('2016-08-15' AS DATE) as ChangeDate
UNION SELECT 'User2' as UserID, 'Team1' AS CurrentTeam, CAST('2016-02-01' AS DATE) as ChangeDate
UNION SELECT 'User2' as UserID, 'Team2' AS CurrentTeam, CAST('2016-07-01' AS DATE) as ChangeDate
)
SELECT *,COALESCE(LEAD(ChangeDate,1) OVER(PARTITION BY UserID ORDER BY ChangeDate),CAST(GETDATE() AS DATE)) as End_Dt
FROM cte
Returns:
UserID CurrentTeam ChangeDate End_Dt
User1 Team1 2016-01-01 2016-06-01
User1 Team2 2016-06-01 2016-08-15
User1 Team1 2016-08-15 2017-01-05
User2 Team1 2016-02-01 2016-07-01
User2 Team2 2016-07-01 2017-01-05
You could then join those ranges to a calendar table to get the individual months as well as calculate which team they spent more days in for a given month.
The LEAD() function returns the next row's value for a given field, PARTITION BY is used to reset the next row based on some grouping, in this case you want the value per UserID, and ORDER BY is used to specify what the next row should be, in this case from one ChangeDate to the next.

Answer 2

You might try this:
--A simple person table
DECLARE #pers TABLE(Person VARCHAR(100));
INSERT INTO #pers VALUES('Bob'),('Tim');
--a table reflecting your work-data
--attention Tim is changing in July to Team Read and still in July back to Blue
DECLARE #Team TABLE(Person VARCHAR(100),Team VARCHAR(100),ChangeDate DATE);
INSERT INTO #Team VALUES
('Bob','Red' ,{d'2016-04-01'})
,('Tim','Blue',{d'2016-04-13'})
,('Tim','Red' ,{d'2016-07-22'})
,('Bob','Blue',{d'2016-06-15'})
,('Tim','Blue',{d'2016-07-28'})
,('Bob','Red' ,{d'2016-10-15'})
,('Tim','Red' ,{d'2016-12-28'})
;
--A CTE to mock-up a numbers/tally/date-table
WITH FirstOfMonthDays(d) AS
(
SELECT {d'2016-01-01'}
UNION ALL SELECT {d'2016-02-01'}
UNION ALL SELECT {d'2016-03-01'}
UNION ALL SELECT {d'2016-04-01'}
UNION ALL SELECT {d'2016-05-01'}
UNION ALL SELECT {d'2016-06-01'}
UNION ALL SELECT {d'2016-07-01'}
UNION ALL SELECT {d'2016-08-01'}
UNION ALL SELECT {d'2016-09-01'}
UNION ALL SELECT {d'2016-10-01'}
UNION ALL SELECT {d'2016-11-01'}
UNION ALL SELECT {d'2016-12-01'}
)
--I use CONVERT(VARCHAR(6),ChangeDate,112) to get a string of YYYYMM
,Numbered AS
(
SELECT ROW_NUMBER() OVER(PARTITION BY Person, CONVERT(VARCHAR(6),ChangeDate,112) ORDER BY ChangeDate DESC) AS Nr
,t.*
FROM #Team AS t
)
--Pick out the one with Nr=1, these are the last changes per month
,LastChangeInMonth AS
(
SELECT *
FROM Numbered
WHERE Nr=1
)
--The actual query
SELECT fom.d
,p.Person
,(
SELECT TOP 1 t.Team
FROM LastChangeInMonth AS t
WHERE t.Person=p.Person
AND CONVERT(VARCHAR(6),t.ChangeDate,112)<=CONVERT(VARCHAR(6),fom.d,112)
ORDER BY t.ChangeDate DESC
) AS fittingTeam
FROM FirstOfMonthDays AS fom
CROSS JOIN #pers AS p
ORDER BY p.Person,fom.d
Since you are using SQL Server 2014 (please tag your questions correctly!) this would be a bit easier with LEAD()/LAG/(), but the idea was the same...
The result
2016-01-01 Bob NULL
2016-02-01 Bob NULL
2016-03-01 Bob NULL
2016-04-01 Bob Red
2016-05-01 Bob Red
2016-06-01 Bob Blue
2016-07-01 Bob Blue
2016-08-01 Bob Blue
2016-09-01 Bob Blue
2016-10-01 Bob Red
2016-11-01 Bob Red
2016-12-01 Bob Red
2016-01-01 Tim NULL
2016-02-01 Tim NULL
2016-03-01 Tim NULL
2016-04-01 Tim Blue
2016-05-01 Tim Blue
2016-06-01 Tim Blue
2016-07-01 Tim Blue
2016-08-01 Tim Blue
2016-09-01 Tim Blue
2016-10-01 Tim Blue
2016-11-01 Tim Blue
2016-12-01 Tim Red