E/R model
sketch databse schemas including constraints.
- Entity set = rectangle
- Attribute = oval, with a line to the rectangle (representing its entity set)
relationship
- connects two or more entity sets.
- represented by a diamonds
value of a relationship is a relationship set
many-to-many relationship
an entity of either set can be connected to many entities of the other set.
many-to-one relationship
each entity of the first set can be connected to at most one entity of the second set. and each entity of the second set can be connected to at least one entity of the first set.