Crow's Foot notation is a practical convention for drawing entity-relationship diagrams. Boxes represent entities, lines represent relationships, and symbols show how many instances may participate in each relationship. section 5.2 section 5.5
Read each relationship line by checking the symbols nearest the related entity:
- The inner symbol shows minimum cardinality: zero or one. section 5.5
- The outer symbol shows maximum cardinality: one or many. section 5.5
- Translate requirements into sentences first, such as “A user has zero, one, or more accounts.” section 5.5
User and account
For a banking database, a user may have zero, one, or many accounts, while each account belongs to one and only one user. The user-to-account direction therefore has minimum zero and maximum many; the account-to-user direction has minimum one and maximum one. Both directions can appear on one relationship line, with each set of symbols placed near its related entity. section 5.5
For a many-to-many relationship, add a junction table containing both entities’ primary keys and represent the result as two one-to-many relationships. A weak entity uses its strong entity’s primary key as part of a composite primary key without a different Crow’s Foot representation. section 5.10 section 5.10
Inspect the diagram figure to connect relationship lines with cardinality symbols and entity structure. section 5.10
