E-R to Relational - Basic
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Transcript of E-R to Relational - Basic
E-R to Relational - Basic
• Strong entity– Table with all attributes of that entity
• Relationships– Table with primary keys of participating entities
• Weak entity– Table with primary key of strong entity
• Multi-valued Attribute X– Table with X, primary key of strong entity– Row per value of X
• Finally ..
E-R to Relational - Improved
• For N-1/1-1 relationships– Move relationship attributes to the N-side entity’s table
• Dept id moves to employee.
• No need for a table for the relationship relating a weak entity– Remove the “depends” table
Emp-Dept ER Diagram
employee
department
dependentcar
Nameaddress
street
Start date
SSN
Dept id name
city
stateLic#
color
name reln
city
Supervises
Manages
Owns Supports
supervisor superviseeWorks for
phones
Emp-Dept schema
• Employee(SSN, name, street, city, deptid, startdate, supervisor_ssn)
• Dept(deptid, name, city, Manager-SSN)
• Phone(SSN, phone#)
• Dependent(EMP-SSN, Name, relation)
• Car(state, license#, color)
• CarOwners(owner-SSN, car-state, license#)