entity relationship model

• ER model - form basis of ER diagram, depict database main components(entity, attributes, relationship)
• entities - correspondence to table but not row in relational environment, in Chen and crow's foot model they are represented by rectangle
• attributes - characteristics of entity, chen model its oval, crow's model its written in attribute box under entity
• domain - attributes have domain and may share a domain, eg: for domain for gender the attributes can be limited to male female or other
• composite attribute can be subdivided(address composed to street, name composed to first name last name), simple attribute cannot be subdivided(salary, date of birth)
• single value attributes - single value
• multivalued attributes - many values
• derived attributes - value may be calculated
• weak entity - entity that does not have a primary key of its own, it relies on the related entity to provide primary key
• relationship degree - indicates number of entities associated with a relationship, unary relationship, binary relationship, ternary relationship, quaternary relationship
• binary relationship called POwns
• ternary relationship called registers
• quaternary relationship called arranges
• recursive relationship - exist between occurrences of same entity set, found in unary relationship
• composite entity - called as bridge entity, eg: in many to many relation, divide to two one to many and the one in the middle is composite entity, separate them