Identifying relationship Non-identifying relationship
symbol a crow’s foot. Relationship lines are frequently labeled to clarify the relationship
and express the rule that the relationship represents. A data model is a communication
tool, and if you think of the entities as nouns, the relationship label is a present tense verb.
Therefore, we are just reading this sentence:
● Each Author may write one or many Titles.
Having a zero in the cardinality makes us use optional-sounding words such as may or can
when reading the relationship. Without the zero, we use mandatory-sounding terms such
as must or have to. So instead of being redundant and saying:
● Each Author may write zero, one, or many Titles.
We take out the word zero because it is expressed using the word may, which implies the
zero:
● Each Author may write one or many Titles.
Every relationship has a parent and a child. The parent entity appears on the one side of
the relationship, and the child appears on the many side of the relationship. In this
example, the parent entity is Author and the child entity is Title. When I read a
relationship, I start with the entity on the one side of the relationship (the parent entity)
first. “Each Author may write one or many Titles.” I then read the relationship from the
many side: “Each Title must be written by one Author.”
I also always use the word each in reading a relationship, starting with the parent side. The
reason for the word each is that you want to specify, on average, how many instances of
one entity relate to an entity instance from the other entity.
There are two types of relationships, identifying and non-identifying. An identifying
relationship is shown with a solid line and means that the entity on the many side (the
child) is always going to be a dependent entity to the entity on the one side (the parent).
The dotted line means that the entity on the many side (the child) is not dependent on the
entity on the one side (the parent). Here are both types of relationships:
Independent entities are displayed as rectangles with sharp corners. Dependent entities,
such as Title under the identifying relationship example, are displayed as rectangles with
rounded corners. An independent entity is an entity where each occurrence (instance) can
be found using only attributes that it owns. For instance, Title under the non-identifying
relationship example can be found by an ISBN (International Standard Book Number),
which is an attribute that belongs to Title. A dependent entity such as Title under the
identifying relationship example can only be found by using at least one attribute from a
different entity such as Author Tax ID from Author.