The Psychology of Quality and More

Entity-Relationship Diagram

Each arrow between data items may be labeled to help clarify the relationship between the two items on either end of the arrow. Other conventions in the diagram below:

• 1 one and only one
• 0,1 = zero or one
• N = from one to many
• 0,N = from zero to many

The diagram shows the cardinality of the relationship. That is, how many of one data items there are for each related item. For example, every person has one mother. A mother, on the other hand, can have a theoretically indefinite number of children.

There are a number of different diagramming conventions to show cardinality. The method below is pretty clear. Other methods include using 'crow's feet' for 'N', a bar across the arrow for '1' and a O across the arrow for a '0'.

The Entity-Relationship Diagram can be used in database design, where it shows how data elements are related. 

See also: