Logical Relationships

Relationships represent connections, links, or associations between entities. They are the verbs of a diagram that show how entities relate to each other. Easy to understand rules help business professionals validate data constraints and ultimately identify relationship cardinality.

Examples of one-to-many relationships:

• A TEAM <has> many PLAYERs
• A PLANE FLIGHT <transports> many PASSENGERs
• A DOUBLES TENNIS MATCH <requires> exactly 4 PLAYERs
• A HOUSE <is owned by> one or more OWNERs
• A SALESPERSON <sells> many PRODUCTs

In all of these cases, the relationships are chosen so that the connection between the two entities is what is known as one-to-many. A one-to-many means that one (and only one instance) of the first entity is related or connected to many instances of the second entity. The entity on the one-end is known as the parent entity. The entity on the many-end is known as the child entity.

Relationships are displayed as a line connecting two entities, with a dot on one end, and a verb phrase written along the line. In the previous examples, the verb phrases are the words inside the brackets, such as <sells>. The following figure shows the relationship between PLANE FLIGHTs and PASSENGERs on that flight: