Selecting from a slowly changing dimension type II

Question

I am having trouble with a slowly changing dimension type II selection. I am looking to select the actual length of an employees skill certification so that i can display start and end of his certification in a report. How would you go about doing that? Below i have included an image of the records for an employee '80844' and below the records the expected result.

I am using Microsoft SQL Server 2008 R2.

Answer 1

Thank you for interesting question. It is well-known 'gaps and islands' problem. You can read more about it there.

For your case solution could look like this:

    create table #tmp 
       (
         dimEmployeeID int not null,
         EmployeeNumber int not null,
         Start datetime not null,
         [End] datetime not null,
         SkillID int not null
       )


   insert into #tmp values
     (386 , 80844, '1900-01-01', '2012-02-28', 14),
     (1172, 80844, '2012-02-29', '2012-02-29', 14),
     (1173, 80844, '2012-03-01', '2012-04-01', 14),
     (1175, 80844, '2012-04-06', '2012-04-12', 14),
     (1176, 80844, '2012-04-13', '2012-04-21', 14),
     (1172, 80844, '2012-02-29', '2012-02-29', 87),
     (1173, 80844, '2012-03-01', '2012-04-01', 87),
     (1174, 80844, '2012-04-02', '2012-04-05', 87),
     (1175, 80844, '2012-04-06', '2012-04-12', 87)

   ;with StartingPoints as
   (
     SELECT EmployeeNumber, SkillID, Start, ROW_NUMBER() OVER(partition by EmployeeNumber,    SkillID order by Start asc) AS rn
     FROM #tmp AS A
     WHERE NOT EXISTS
       (
         SELECT 1
         FROM #tmp AS B
         WHERE 
           A.EmployeeNumber = B.EmployeeNumber
           and A.SkillID = B.SkillID
           and A.Start - 1 = B.[End]
        )
   ),
   EndingPoints AS
   (
     SELECT EmployeeNumber, SkillID, [End], ROW_NUMBER() OVER(partition by EmployeeNumber,    SkillID order by Start asc) AS rn
     FROM #tmp AS A
     WHERE NOT EXISTS
       (
         SELECT 1
         FROM #tmp AS B
         WHERE 
           A.EmployeeNumber = B.EmployeeNumber
           and A.SkillID = B.SkillID
           and A.[End] + 1 = B.Start
        )
   )
   SELECT S.EmployeeNumber, S.SkillID, S.Start, E.[End]
   FROM StartingPoints AS S
     JOIN EndingPoints AS E
   ON 
     S.EmployeeNumber = E.EmployeeNumber
     and S.SkillID = E.SkillID
     and S.rn = E.rn