Entities with a strict 1-2 relationship

Question

I am trying to understand how do we model and define the cardinality of two entities when the relationship is of exact nature. Lets say 1 to 2.

For an example, if I have entities Journey and Location. Now each journey has 2 locations, a starting location and an ending location.

So how do we model this relationship and put the cardinality on each entity (Journey and Location).

Simple google search results in simple 1-1, 1-m, and m-n relationship explanations. But how do we limit them to be exactly 1-2 ?

Answer 1

Your example is not "1 to 2", but "N to 2", and you'd model it simply by having two foreign keys:

(And probably a CHECK to ensure that starting and ending locations differ.)

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If you really wanted "1 to 2", in a sense that any given location is always connected to just one journey, you'd have to do something like this...

...and use a DBMS that supports deferred constraints, so you can break chicken-and-egg problem when inserting new data.

On a DBMS that doesn't support deferred constraints, you could make starting and ending locations NULL-able, but then it would no longer be "1 to 2", but "1 to 0..2" relationship.