IT420: Database Management and Organization Normalization 31 January 2006 Adina Crăiniceanu

Kroenke, Database Processing Previously on IT420  Database design  Entity-Relationship (ER) Model  Relational Model  Transform ER to relational model  Database Implementation  SQL

Kroenke, Database Processing Goal  Refresh ER to Relational  Normalization

Kroenke, Database Processing ER to Relational  ER:  Entities  identifiers  Relationships  cardinality  Relational model  Tables  Constraints

Kroenke, Database Processing ER to Relational  Transform entities in tables  Transform relationships using foreign keys  Specify logic for enforcing minimum cardinalities

Kroenke, Database Processing Class Exercise: Transform ER model into Relational Model

Kroenke, Database Processing Outline  ER to Relational  Normalization

Kroenke, Database Processing Premise  We have received one or more tables with data  The data is to be stored in a new database  QUESTION: Should the data be stored as received, or should it be transformed for storage?

Kroenke, Database Processing Data Redundancy NumberLastNameFirstName RatingWage Application constraint: All employees with same rating have the same wage (Rating  Wage) Problems due to data redundancy?

Kroenke, Database Processing Modification Anomalies  Deletion Anomaly: What if we delete all employees with rating 8?  Insertion Anomaly: What if we have a new employee with new rating 12?  Update Anomaly: What if we change the wage for rating 7 to be 27?

Kroenke, Database Processing Update Anomalies  The EMPLOYEE table before and after an incorrect update operation on Wage for Rating = 7 NumberLastNameFirstName RatingWage NumberLastNameFirstName RatingWage

Kroenke, Database Processing Table decomposition NumberLastNameFirstName RatingWage NumberLastNameFirstName Rating RatingWage Problem?

Kroenke, Database Processing Decisions  Do we have to decompose / merge?  How do we identify problems caused by redundancy?  Functional dependencies

Kroenke, Database Processing Functional Dependency (FD)  A functional dependency: the value of one (a set of) attribute(s) determines the value of a second (set of) attribute(s): Alpha  MIDNName Alpha  (DormName, DormRoom)  The attribute on the left side of the functional dependency is called the determinant  Functional dependencies may be based on equations: Charge = NbHours X HourlyPrice (NbHours, HourlyPrice)  Charge  Function dependencies are not equations!

Kroenke, Database Processing Functional Dependencies Are Not Equations ObjectColor  Weight ObjectColor  Shape ObjectColor  (Weight, Shape)

Kroenke, Database Processing Composite Determinants  Composite determinant: A determinant of a functional dependency that consists of more than one attribute  (StudentName, ClassName)  Grade  (NbHours, HourlyPrice)  Charge

Kroenke, Database Processing Functional Dependency (FD) Rules  If A  (B, C), then A  B and A  C  If (A,B)  C, then neither A nor B determines C by itself

Kroenke, Database Processing FD Facts  A functional dependency is a statement about all allowable instances of a table  You cannot find the functional dependencies simply by looking at some data:  Data set limitations  Must be logically a determinant  Given some data in a table R, we can check if it violates some FD, but we cannot tell if the FD holds over R!

Kroenke, Database Processing Functional Dependencies in the SKU_DATA Table Assuming data is representative, determine the FD

Kroenke, Database Processing Functional Dependencies in the SKU_DATA Table SKU  (SKU_Description, Department, Buyer) SKU_Description  (SKU, Department, Buyer) Buyer  Department

Kroenke, Database Processing Class Exercise Assuming all FD that are verified by the data are indeed FD, determine the FD in the ORDER_ITEM Table

Kroenke, Database Processing What Makes Determinant Values Unique?  A determinant is unique in a relation if, and only if, it determines every other column in the relation  Unique determinants = superkey

Kroenke, Database Processing Key  A set of columns is a key for a relation if : 1. No two distinct rows can have same values in all key columns, and 2. This is not true for any subset of the key  Part 2 false? A superkey  Primary key  Alternate key

Kroenke, Database Processing Normal Forms  Relations are categorized as a normal form based on which modification anomalies or other problems that they are subject to:

Kroenke, Database Processing Normal Forms  Boyce-Codd Normal Form (BCNF) – A relation is in BCNF if every determinant is a candidate key “I swear to construct my tables so that all nonkey columns are dependent on the key, the whole key and nothing but the key, so help me Codd.”

Kroenke, Database Processing Eliminating Modification Anomalies from Functional Dependencies in Relations  Put all relations into Boyce-Codd Normal Form (BCNF):

Kroenke, Database Processing Putting a Relation into BCNF: EQUIPMENT_REPAIR

Kroenke, Database Processing Putting a Relation into BCNF: EQUIPMENT_REPAIR EQUIPMENT_REPAIR (ItemNumber, Type, AcquisitionCost, RepairNumber, RepairDate, RepairAmount) ItemNumber  (Type, AcquisitionCost) RepairNumber  (ItemNumber, Type, AcquisitionCost, RepairDate, RepairAmount) ITEM (ItemNumber, Type, AcquisitionCost) REPAIR (ItemNumber, RepairNumber, RepairDate, RepairAmount) Where REPAIR.ItemNumber must exist in ITEM.ItemNumber

Kroenke, Database Processing Putting a Relation into BCNF: New Relations

Kroenke, Database Processing Class Exercise Put the following relation into BCNF

Kroenke, Database Processing Multivalued Dependencies  A multivalued dependency occurs when a determinant determines a particular set of values: Employee  Degree Employee  Sibling PartKit  Part  The determinant of a multivalued dependency can never be a primary key

Kroenke, Database Processing Multivalued Dependencies

Kroenke, Database Processing Eliminating Anomolies from Multivaled Dependencies  Multivalued dependencies are not a problem if they are in a separate relation, so:  Always put multivalued dependencies into their own relation  This is known as Fourth Normal Form (4NF)