Database Relationships: an overview

The three kinds of relationship between tables (one-to-many, one-to-one and many-to-many) and how a diagram shows each.

Apprentice · SQL · Database · Updated

Tables are linked to each other through relationships. There are three different types of theoretical relationships in a database.

One-to-many in crow's foot notation: a single bar at the parent end, a crow's foot at the child end

  • One-to-Many: a row in a parent table can be related to many rows in a related child table

One-to-one in crow's foot notation: a single bar at both ends

  • One-to-One: a row in a parent table can be related to only one row in a related child table

Many-to-many in crow's foot notation: a crow's foot at both ends

  • Many-to-Many: multiple rows in one table can be related to multiple rows in another. In this scenario, both tables act as both parent and child.

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