T-SQL Query to remove duplicate records in the output based on one particular column - sql-server

Question

I am running SQL Server 2014 and I have the following T-SQL query:
USE MYDATABASE
SELECT *
FROM RESERVATIONLIST
WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')
RESERVATIONLIST mentioned in the code above is a view. The query gives me the following output (extract):
ID NAME DOA DOD Nights Spent MTH
--------------------------------------------------------------------
251 AH 2015-01-12 2015-01-15 3 JANUARY 2015
258 JV 2015-01-28 2015-02-03 4 JANUARY 2015
258 JV 2015-01-28 2015-02-03 2 FEBRUARY 2015
The above output consist of around 12,000 records.
I need to modify my query so that it eliminates all duplicate ID and give me the following results:
ID NAME DOA DOD Nights Spent MTH
--------------------------------------------------------------------
251 AH 2015-01-12 2015-01-15 3 JANUARY 2015
258 JV 2015-01-28 2015-02-03 4 JANUARY 2015
I tried something like this, but it's not working:
USE MYDATABASE
SELECT *
FROM RESERVATIONLIST
WHERE [MTH] IN ('JANUARY 2015', 'FEBRUARY 2015')
GROUP BY [ID]
HAVING COUNT ([MTH]) > 1

Answer 1

Following query will return one row per ID :
SELECT * FROM
(
SELECT *,ROW_NUMBER() OVER (PARTITION BY ID ORDER BY (SELECT NULL)) rn FROM RESERVATIONLIST
WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')
) T
WHERE rn = 1
Note : this will return a random row from multiple rows having same ID. IF you want to select some specific row then you have to define it in order by. For e.g. :
SELECT * FROM
(
SELECT *,ROW_NUMBER() OVER (PARTITION BY ID ORDER BY DOA DESC) rn FROM RESERVATIONLIST
WHERE [MTH] IN ('JANUARY 2015','FEBRUARY 2015')
) T
WHERE rn = 1
definitely, it will return the row having max(DOA).

Answer 2

You are trying to do a GROUP BY statement which IMHO is the right way to go. You should formulate all columns that are a constant, and roll-up the others. Depending on the value of DOD and DOA I can see two solutions:
SELECT ID,NAME,DOA,DOD,SUM([Nights Spent]) as Nights,
min(MTH) as firstRes, max(MTH) as lastRes
FROM RESERVATIONLIST
GROUP BY ID,NAME,DOA,DOD
OR
SELECT ID,NAME,min(DOA) as firstDOA,max(DOD) as lastDOD,SUM([Nights Spent]) as Nights,
min(MTH) as firstRes, max(MTH) as lastRes
FROM RESERVATIONLIST
GROUP BY ID,NAME