I am having trouble grasping the "relationship" concept of an entity-relationship diagram. For example, you have Classroom and Furniture as your entities. If you were to put a relationship diamond with the word Contains and a line between the two, how would one be able to tell which contains which?

Obviously we know that Furniture cannot contain a Classroom, however, when it comes to applications and data models there are often foreign concepts and terms meaning we have no prior knowledge of the relationship or possibilities.

Without actually having words to describe the diagram, which would defeat the purpose of the ERD (as it should speak for itself), how would one determine visually which way the relationship was flowing?

Thanks in advance.

FYI; in case it is relevant - the line touching Classroom has a "one and only one" symbol, the line touching Furniture has a "zero through many" symbol.

2 Answers 2


The flow of ERD relationships go in both directions. Cardinality expresses the rules governing the relationships between entities. The 'one and only one' of classroom provides you with a verbal logic of - 'A classroom must contain..'. The 'zero to many' furniture cardinality provides 'zero to many pieces of furniture.'

Don't think of the furniture physically being in the classroom, just that a connection exists between the two.

  • It seems silly to me and I am really struggling to grasp why there isn't at least a directional arrow on either side of the relationship diamond. Fact of the matter is a lot of ERDs I see, I can tell by looking at the entities which way the relationship goes, however, for a lot I also can't tell. E.g. Entity: Training Package, Entity: Course, Relationship: Contains. I doubt without looking into the system you would be able to tell which way that relationship is going...
    – applaps
    Commented Feb 7, 2018 at 23:37
  • Also, if the 'one and only one' is touching classroom in my above example, doesn't that say that Furniture can only have a relationship with one classroom? A classroom doesn't have to have furniture, but furniture cannot exist in more than one classroom..
    – applaps
    Commented Feb 7, 2018 at 23:42
  • 2
    ER diagrams can have arrows. Or numbers indicating whether a relation is on the 1 or the many on a 1-to-many relationship. So, it could have fr example Classroom 1..1 and Furniture 0..n in your case Commented Feb 8, 2018 at 2:09
  • hmm this still doesn't help me... It really just sounds like relationships are silly unless you have prior knowledge of the entities you are looking at in the ERD, which kind of makes them kind of pointless...
    – applaps
    Commented Feb 9, 2018 at 5:24

I think this will help you and other who have the same issue.

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