How to address a multiple purpose column in sql? - sql

Question

Scenario
I need to design a equipment movement from many sources and destinations. I have the folowing sample tables:
CREATE TABLE Area{
Id INT,
Name VARCHAR(50),
//some other fields
}
CREATE TABLE Stowage{
Id Int,
Name VARCHAR(50),
//some other fields
}
CREATE TABLE Movement{
OriginId INT,
DestinationId INT,
}
But I need some kind of movement like:
Origin : Area; Destination: Area
Origin : Area; Destination: Stowage
Origin : Stowage; Destination: Area
Origin : Stowage; Destination: Stowage
But I only have two columns and needs more than one foreign key per column.
Posible solution in mind
Create MovementArea, MovementStowage, MovementStowageArea tables and create its propertly foreigns keys.
Don't create foreigns key for the columns OriginId and DestinationId and fill it as needed.
Final Question
Is there another way to address this in sql or which of the provided solutions is most aceptable for this scenario?

Answer 1

Tricky. You have 4 foreign keys, so I would [naturally] create 4 foreign key columns, as in:
create table movement (
origin_area int,
origin_stowage int,
dest_area int,
dest_stowage int,
constraint fk1 foreign key origin_area references area (id),
constraint fk2 foreign key origin_stowage references stowage (id),
constraint fk3 foreign key dest_area references area (id),
constraint fk4 foreign key dest_stowage references stowage (id),
constraint chk_fk1 check (origin_area is null and origin_stowage is not null
or origin_area is not null and origin_stowage is null),
constraint chk_fk2 check (dest_area is null and dest_stowage is not null
or dest_area is not null and dest_stowage is null)
);
Now, as you see:
There are 4 nullable FK columns.
Each FK column has its corresponding FK constraint.
Also, origin_area and origin_stowage are mutually exclusive. Always one of them is null, while the other points to the other table. This is enforced by the constriaint chk_fk1.
The same can be said for dest_area and dest_stowage. Enforced by chk_fk2.

Answer 2

My first thought is something like this:
Create Table MovementEndpoint
(
ID Int
, Name Varchar(50)
, EndpointType Int -- Area, Stowage, etc
, EndpointDetailID Int -- FK to Area, Stowage, etc
)
Now your movements just go between endpoints, and the MovementEndpoint record lets you get to the Area or Stowage record as needed. The query logic will still be a little tricky, but no more so than your initial design requires.