Data Cardinality

I'm confusing about 0:M relationship.

So, I want to ask about that.

Assumes we have 2 tables :

Contact which has attributes like : ContactID(PK), Name

Address which has attributes like : AddressID(PK), Desc, ContactID(FK to Contact, Nullable, No开发者_StackOverflow社区t Unique)

Is my statement right that :

• The 0:M relationship occurred when there is 1 row in Contact and it's ContactID don't appear in Address.

• To make the 0:M relationship Column ContactID in Table Address must be nullable.

Thanks in advance

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• The 0:M notation is used to annotate one side of a relationship, indicating its cardinality. It is a range and is accurate in every instance of the relationship. When ContactID doesn't occur in address, the cardinality is 0.

• Foreign keys don't need to be nullable, and shouldn't in general. The 0 case still happens, when a ContactID occurs in Contact and not Address.