Weak entity set:
no primary key.
Identified entity set:
related to weak entity set.
Strong entity set: entity set
with primary key.
Weak entity set cannot be the primary key independently.
Multi-valued attributes are attributes that can have multiple values for an entity. For example: a student information table has an attribute about interest. A student may have several interests, such as sports, movies, travel, learning, etc. These students may only have "sports" interests, or have "sports" at the same time. , Movies, travel, and learning ", so interest is a multi-valued attribute.
Double-line rectangle: weak entity set
Underline dashed line: discriminator
Double-line diamond: symbolic connection
Cardinality Constraints
One
-
to
- one One - to -many Many - to - many Many - to - many
Participation of an Entity Set in a Relationship Set
Partial participation
Full participation in
Redundancy of Schemas
multi-valued attributes: a new table should be created separately
Aggregation
