SQL Server : find break in dates to show unique rows - sql

Question

I have developed a solution to a problem (I think), and I am keen to see if there is a better way around this, as I can't help but feel there is a better way.
The problem: a company name, and a move in date are shown. The company could leave, another company come in and then the original company could come back. To make this problem a bit tricky, there may be rogue dates for a company in there. Best way to explain it is via the table:
Table example
What I need to extract, is only the first time a company moved in, until it is broken by a different company and so on.
The code I have is:
IF OBJECT_ID('tempdb..#tmpData') IS NOT NULL
DROP TABLE #tmpData
GO
CREATE TABLE #tmpData
(
COMPANY_NAME NVARCHAR(30),
DATE_MOVED_IN DATETIME,
ID INT IDENTITY(1,1),
UNIQUE_ID INT
)
INSERT INTO #tmpData(COMPANY_NAME, DATE_MOVED_IN)
SELECT 'ABC LTD','01/01/2017' UNION ALL
SELECT 'ABC LTD','01/04/2017' UNION ALL
SELECT 'XYZ LTD','01/10/2017' UNION ALL
SELECT 'ABC LTD','01/12/2017';
DECLARE #intMinID INT,
#intMaxID INT,
#strNextComp NVARCHAR(50),
#strCurrentComp NVARCHAR(50),
#strPreviousComp NVARCHAR(50),
#intMaxUID INT;
SELECT
#intMinID = MIN(TD.ID),
#intMaxID = MAX(TD.ID)
FROM
#tmpData AS TD
UPDATE TD
SET TD.UNIQUE_ID = 1
FROM #tmpData AS TD
WHERE TD.ID = #intMinID;
WHILE #intMinID <= #intMaxID
BEGIN
SELECT
#strCurrentComp = TD.COMPANY_NAME
FROM
#tmpData AS TD
WHERE
TD.ID = #intMinID;
SELECT
#strNextComp = TD.COMPANY_NAME
FROM
#tmpData AS TD
WHERE
TD.ID = (#intMinID + 1)
SELECT
#strPreviousComp = CASE WHEN EXISTS (SELECT 1
FROM #tmpData AS TD
WHERE TD.ID = (#intMinID - 1))
THEN TD.COMPANY_NAME
ELSE 'No Company Exists'
END
FROM
#tmpData AS TD
WHERE
TD.ID = (#intMinID - 1)
SELECT
#intMaxUID = MAX(TD.UNIQUE_ID)
FROM
#tmpData AS TD
IF(#strPreviousComp IS NULL)
PRINT 'Nothing to do'
ELSE IF((#strCurrentComp <> #strNextComp) AND (#strCurrentComp = #strPreviousComp))
BEGIN
UPDATE TD
SET TD.UNIQUE_ID = #intMaxUID
FROM #tmpData AS TD
WHERE TD.ID = #intMinID;
END
ELSE
BEGIN
UPDATE TD
SET TD.UNIQUE_ID = #intMaxUID + 1
FROM #tmpData AS TD
WHERE TD.ID = #intMinID;
END
SET #intMinID = #intMinID + 1;
END
SELECT
COMPANY_NAME, MIN(DATE_MOVED_IN) AS DATE_MOVED_IN
FROM
#tmpData
GROUP BY
COMPANY_NAME, UNIQUE_ID
ORDER BY
UNIQUE_ID ASC
Any suggestions on how to do this in a more efficient way, or if any errors are spotted, feedback is very much appreciated.
Thanks,
Leo

Answer 1

Lag() should do it...
with CTE as
(
select Company_Name, Date_Moved_in, lag(Company_Name) over (order by Date_Moved_In) as PrevComp
from #TempTable
)
select Company_Name, Date_Moved_In
from CTE
where PrevComp <> Company_Name
or PrevComp is null

Answer 2

You can use the difference in row number logic to classify continuous dates by company into one group. Run the inner query alone to see how groups are assigned.
Thereafter, just group by the company and previously classified group to get the first date moved in.
select company_name,min(date_moved_in)
from (
select t.*,
row_number() over(order by date_moved_in)
-row_number() over(partition by company_name order by date_moved_in) as grp
from #tmpData t
) x
group by company_name,grp