Question

I was given a database design which stores information about an organization and any changes that have happened or will happen to the organizations. Since pretty much anything can change about an organization, there is one table that only contains the unique OrganizationID’s in a table called ‘Organizations’. The changes to that organization all have an effective date and follow a similar design pattern such as the following, the location change:

Table: organization_locations  organization_id (int, not null) - Relates back to the Organizations.ID column. location_id (int, not null) - Relates to Locations.ID eff_date (datetime, not null) - The date this change becomes effective  Table: Locations  ID (int, pk, identity, not null) - ID of the location Name (varchar(255), not null) - Name of the location ... Other miscellaneous columns that aren't important for this discussion ... 

E.G. Organizations may just contain two rows which would simply hold the id’s 1 and 2 respectively. Locations may have 3 locations (id, name):

1, Location1 2, Location2 3, Location3  organization_locations (organization_id, location_id, eff_date):  1, 1, 1/1/2000  <--- Organization 1 is starting at location 1 1, 2, 1/1/2010  <--- On 1/1/2010, organization 1 moves to location 2 (from location 1) 1, 3, 1/1/2011 <--- On 1/1/2011, organization 1 moves to location 3 (in this case from location 2) 

I already have a large and possibly overly complex query for specifying a date and getting back an organizations status at that given time but I feel like there’s probably an easier way. However, the problem at hand is the following:

From this schema, how can I answer a question such as ‘What organizations will move from Location 1 to another location and what organizations will move to Location 1 from another location in the given timeframe: date1 to date2?’

A similar question that could also answer the first is: How can I query each organization’s location changes (easy) while showing the PREVIOUS location they are moving from (hard?) ?

Note: Including the LINQ tag in case there’s an easy way to do it in LINQ I can go that route.

Answer 1

To Location 1:

SELECT  DISTINCT organization_id FROM    organization_locations ol WHERE   ol.eff_date BETWEEN @date1 AND @date2         AND ol.location = 1 

From Location 1:

SELECT  DISTINCT organization_id FROM    (         SELECT organization_id,                (                SELECT  TOP 1 location_id                FROM    organization_locations oln                WHERE   oln.organization_id = ol.organization_id                        AND oln.eff_date < ol.eff_date                ORDER BY                        organization_id DESC, eff_date DESC                ) AS previous_location         FROM   organization_locations ol         WHERE  eff_date BETWEEN @date1 AND @date2         ) olo WHERE   previous_location = 1 

Show previous location:

SELECT ol.*,        (        SELECT  TOP 1 location_id        FROM    organization_locations oln        WHERE   oln.organization_id = ol.organization_id                AND oln.eff_date < ol.eff_date                AND location_id = 1        ORDER BY                organization_id DESC, eff_date DESC        ) AS previous_location FROM   organization_locations ol 

Having a UNIQUE INDEX on (organization_id, eff_date) will help you a lot.