Cơ sở dữ liệu - Normalization for relational databases

Normalization for Relational Databases Informal Design Guidelines for Relational Databases  Relational database design: The grouping of attributes to form "good" relation schemas  Two levels of relation schemas: ◦ The logical "user view" level ◦ The storage "base relation" level  Design is concerned mainly with base relations  Criteria for "good" base relations: ◦ Discuss informal guidelines for good relational design ◦ Discuss formal concepts of functional dependencies and normal forms 1NF 2NF 3NF BCNF Semantics of the Relation Attributes  Each tuple in a relation should represent one entity or relationship instance ◦ Only foreign keys should be used to refer to other entities ◦ Entity and relationship attributes should be kept apart as much as possible ◦ Design a schema that can be explained easily relation by relation. The semantics of attributes should be easy to interpret. Redundant Information in Tuples and Update Anomalies  Mixing attributes of multiple entities may cause problems ◦ Information is stored redundantly wasting storage ◦ Problems with update anomalies:  Insertion anomalies  Deletion anomalies  Modification anomalies EXAMPLE OF AN UPDATE ANOMALY Consider the relation: EMP_PROJ ( Emp#, Proj#, Ename, Pname, No_hours) ◦ Update Anomaly  Changing the name of project number P1 from “Billing” to “Customer-Accounting” may cause this update to be made for all 100 employees working on project P1 ◦ Insert Anomaly  Cannot insert a project unless an employee is assigned to .  Inversely- Cannot insert an employee unless he/she is assigned to a project. EXAMPLE OF AN UPDATE ANOMALY (2) ◦ Delete Anomaly  When a project is deleted, it will result in deleting all the employees who work on that project. Alternately, if an employee is the sole employee on a project, deleting that employee would result in deleting the corresponding project.  Design a schema that does not suffer from the insertion, deletion and update anomalies. If there are any present, then note them so that applications can be made to take them into account Null Values in Tuples  Relations should be designed such that their tuples will have as few NULL values as possible ◦ Attributes that are NULL frequently could be placed in separate relations (with the primary key) ◦ Reasons for nulls: ◦ a. attribute not applicable or invalid ◦ b. attribute value unkown (may exist) ◦ c. value known to exist, but unavailable Introduction to Normalization  Normalization: Process of decomposing unsatisfactory "bad" relations by breaking up their attributes into smaller relations  Normal form: Condition using keys and FDs of a relation to certify whether a relation schema is in a particular normal form ◦ 2NF, 3NF, BCNF based on keys and FDs of a relation schema ◦ 4NF based on keys, multi-valued dependencies First Normal Form  Disallows composite attributes, multivalued attributes, and nested relations; attributes whose values for an individual tuple are non-atomic  Considered to be part of the definition of relation First Normal Form  Multi valued attributes ◦ Create more tuples in the relation. ◦ Create a new relation with foreign keys to this relation  Composite attributes ◦ Create an attribute for each sub-attribute. Non-atomic Second Normal Form  Uses the concepts of FDs, primary key  Definitions: ◦ Prime attribute - attribute that is member of the primary key K ◦ Full functional dependency - a FD Y  Z where removal of any attribute from Y means the FD does not hold any more Examples Second Normal Form  1NF + no partial dependency  Partial dependency ◦ An attribute is dependent of a PART of the key. ◦ Only interesting if the key is composite.  What to do? Divide the relation ◦ Partial dependent attributes must have their own relation. Examples Second Normal Form  clientNo, propertyNo  pAddress, rentStart, rentFinish, rent, ownerNo, oName IS NOT FULL FD since propertyNo  pAddress, rent, ownerNo, oName 1NF propertyNo → pAdress PK = {clientNo, propertyNo) 1NF 2NF Third Normal Form A relation schema R is in third normal form (3NF) if it is in 2NF and no non- prime attribute A in R is transitively dependent on the primary key 3rd Normal Form : 1. propertyNo  pAddress, rent, ownerNo, oName  propertyNo  ownerNo 2. ownerNo  oName : propertyNo  oName  PropertyOwner không đạt 3NF BCNF (Boyce-Codd Normal Form)  A relation schema R is in Boyce-Codd Normal Form (BCNF) if whenever an FD X  A holds in R, then X is a superkey of R ◦ Each normal form is strictly stronger than the previous one:  Every 2NF relation is in 1NF  Every 3NF relation is in 2NF  Every BCNF relation is in 3NF ◦ There exist relations that are in 3NF but not in BCNF ◦ The goal is to have each relation in BCNF (or 3NF) 3NF Primary key Primary key BCNF  {Student,course}  Instructor  Instructor  Course  Decomposing into 2 schemas ◦ {Student,Instructor} {Student,Course} ◦ {Course,Instructor} {Student,Course} ◦ {Course,Instructor} {Instructor,Student} Example  Given the relation Book(Book_title, Authorname, Book_type, Listprice, Author_affil, Publisher) The FDs are Book_title  Publisher, Book_type Book_type  Listprice Authorname Author_affil