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entity-relationship

Interpreting the Crow's foot ER symbols

I came to know that there is Crow's foot ER diagram notation, and got a good quick reference. Have a look at the below diagram:

enter image description here

I did get some clarity on these symbols, however I have difficulty in understanding:

1) Why we have the "connector symbols" on both side of the line?

2) How to interpret them?

Can anyone please help me understand this?

Solution

  • Your reference is somewhat incorrect. The Entity-Relationship model doesn't use Crow's foot. Chen's notation and extensions to that notation can be called ER diagrams.

    Your diagram is a generalization of table diagrams, modified to allow many-to-many associations and hide attributes. However, it doesn't represent relationships using their own shapes, and I see no indication of support for ternary or higher relationships. There's also no indication of other ER concepts like weak entities, associative entities, identifying relationships or keys.

    Data models that only support binary relationships are usually based on the network data model, not on the Entity-Relationship model. Relationships in the ER model, when physically implemented, are represented by two or more entity columns in the same table, not by any kind of link between tables. In actual table diagrams, many-to-many relationships are represented by their own table, with two one-to-many association lines. In those cases, the cardinality indicators represent the number and optionality of records with matching values for matching PK/FK columns.

    The interpretation of the Crow's foot symbols are indicated in the orange column. For more information about modeling with Crow's foot, see Entity Modelling. However, don't confuse this with the Entity-Relationship model as described by Chen, or either of them with the Relational model.