Dbms chapter vii

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Dbms chapter vii

  1. 1. Subject Name Code Credit Hours Database System COMP 219 3 Chapter VII
  2. 2. Subject Name Code Credit Hours Database System COMP 219 3 Relational Model: • It uses a collection of tables to represent both data and relationships among those data. Basic Structure  In this model each table is called as relation. Each relation has a no. of row is called tuples and columns called as fields or attributes.
  3. 3. Subject Name Code Credit Hours Database System COMP 219 3 Relational Model: • For an attribute, a range of permitted values can be specified. That range is called as Domain of that attribute. • E.g.. Age – attribute • Domain (Age) = {1,2…100}
  4. 4. Subject Name Code Credit Hours Database System COMP 219 3 Relational Model: • A table of n attributes must be a subset of D1*D2*…….Dn • E.g.. Eid Ename Eaddr 1 2 3 Aaa Bbb ccc South street West street New Employee Relation
  5. 5. Subject Name Code Credit Hours Database System COMP 219 3 Relational Model: • To denote the value of attribute ‘a’ on second tuple, then it can be denoted as t2[a]=value. • E.g.. • t2[ename]=“bbb”.
  6. 6. Subject Name Code Credit Hours Database System COMP 219 3 Relation schema • It is the description about the structure of the relation and the relation instance is the snapshot of data in the relation at a given instant in time. • E.g.. Employee_schema=(eid,ename,eaddr) Name of the schema
  7. 7. Subject Name Code Credit Hours Database System COMP 219 3 Relation schema • Empoyee(Employee_schema) denote that • Employee is a relation on Employee_schema
  8. 8. Subject Name Code Credit Hours Database System COMP 219 3 Keys • Strong Entity set: • The Primary key of the entity set becomes the primary key of the relation. • Weak Entity Set: • The table corresponding a weak entity set includes • the attributes of the weak entity set & • primary key of the strong entity set on which the weak entity set depends.
  9. 9. Subject Name Code Credit Hours Database System COMP 219 3 Keys Relationship Set: The union of the primary key of the related entity sets becomes a super key of the relation. Combined Tables: If there exists many to one relationship set from A to B, then the prim.key of A becomes the prim.key of the relation & the relation consists of the attribues of A and attributes of the relationship set.
  10. 10. Subject Name Code Credit Hours Database System COMP 219 3 Keys Foreign Key: An attribute of a relation r1 that references relation r2 is called as foreign key attribute. Schema Diagram: A relation schema can be depicted pictorially by schema diagram,
  11. 11. Subject Name Code Credit Hours Database System COMP 219 3 Keys • E.g.. eid Ename eaddr dno dnamedno dloc Employee pnamepno eid Dept Project
  12. 12. Subject Name Code Credit Hours Database System COMP 219 3 Query Languages: • It is a lang in which a user requests information from db. Query Languages Procedural Language Non Procedural Language Specifies what data are required & specify how to get those data e.g Relational Algebra Specifies what data are required without specify how to get those data e.g. Relational calculus

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