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sql-serverdatabasenullforeign-keysdiagram

ERDs with Nulls and one to one relationship

I want to know if in this relationship, I am saying that the foreign key MemberID can be set to null in the Marketing Target table?

Does this mean that each MTid has one and only one MemberID but the MemberID can also be null?

I'm confused on how to read the FK and PK here.

Thanks!

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Solution

  • This is crows foot notation. There are two components at each of the relationship - the outer (the one closest to the table) and the inner one.

    The inner one can either be a circle or a dash intersecting the line - in the example above you have one of each at either end. A circle means that end of the relationship is optional. The dash at the other end means it is required.

    The outer one can either be a dash or a fanout three lines (crows feet). The dash means this is a single record, and the crows feet (not shown above) means many.

    So your diagram should be read as follows: Each Member details can have none or one MarketingTarget, and each MarketingTarget MUST have one and only one matching MemberDetails record. in other words, the MarketingTarget.MemberID field cannot be NULL, but, if there is a value in it, it must be unique. Uniqueness due to the Dash-Circle on the left rather than a crowsfoot-circle.