in-house records over outside records - sql

Question

I am stuck with this problem.
I have duplicates in my query coming from that some of the records are in-house and outside simultaneously. I prefer in-house over outside but some of the outside are preferable when there is no entrance from in-house.
Select id, date, location from prod
id date location
----------
2 01/01/2012 in-house
2 05/01/2012 outside <- in this situation i want to keep just in-house
id date location
----------
4 01/01/2012 in-house
5 03/01/2012 outside <- in this situation i want to keep both since there is no db entry for id=5 therefor i have just info from outside
Could someone help?

Answer 1

One way to do this is to do a full outer join from your table to itself and then use coalesce.
Select
COALESCE(Inside.Id, outside.id) Id,
COALESCE(Inside.date, outside.date) Date,
COALESCE(Inside.location, outside.location) Location
From
prod Inside
FULL OUTER JOIN prod Outside
ON Inside.id = Outside.iD
and Inside.location <> Outside.Location
Where
(Inside.Location = 'in-house'
or
Inside.Location is null)
and
(outside.Location = 'outside'
or
outside.Location is null)
DEMO
Notes:
If your fields can be nullable you may want to use a Case statement instead of coalesce and use the ID field to determine which table to use. Using the date as an example
CASE WHEN Inside.Id is not null THEN Inside.date ELSE outside.date END date
As Dems noted this also assumes that {id, location} is unique.
UPDATE
Since you're using SQL Server and {ID, Location} isn't unique and you want the max date value and always choosing in-house over outside then you can use ROW_NUMBER/WHERE RowNumber = 1 effectively here, by ordering First on location and then by date.
WITH cte
AS (SELECT Row_number() OVER ( partition BY ID
ORDER BY CASE LOCATION WHEN 'in-house' THEN 0
WHEN 'outside' THEN 1 END,
DATE DESC) rn,
ID,
Date,
Location
FROM prod)
SELECT ID,
Date,
Location
FROM cte
WHERE rn = 1
Demo
Note We didn't have to use a case statement but I wanted the mapping to be explicit.