Normalization A337

A337 - Reed Smith2 Structure What is a database? ◦ Tables of information  Rows are referred to as records  Columns are referred to as fields or attributes  Record identifier is referred to as a record key Types ◦ Relational - Most common, Object-Oriented ◦ Hierarchical, Network (much older types)

A337 - Reed Smith3 Database structure Two approaches to the structure issue: ◦ Conceptual (you start with the question of “what information should I have?”)  ERD from “scratch” ◦ Empirical (you already know what data there will be - you just want to organize it into tables) –  NORMALIZATION

Database Tables and Normalization Normalization - Process for evaluating and correcting table structures to minimize data redundancies ◦ Works through a series of stages called normal forms:  Normal form (1NF)  Second normal form (2NF)  Third normal form (3NF) ◦ There are higher forms but are rarely necessary

A337 - Reed Smith5Normalization Why? ◦ Data structures need to:  Minimize redundancy  Avoid insertion, update, and deletion anomalies How? ◦ Restructure information such that:  Only flat (rectangular) files exist (1st normal form) – No Nulls  All items in each record depend upon) the primary record key (2nd normal form) – No Partial Dependencies  If a field depends upon another then the “other” must be a primary key (3rd normal form) – No Transitive Dependencies

A337 - Reed Smith6 Normalize the following table:

A337 - Reed Smith7 What is wrong with this solution?

First Normal Form Eliminate Nulls/Repeating Groups – ◦ Eliminate repeating groups by eliminating nulls, filling in cells with implied values with actual values Select a primary key ◦ may be a composite key

A337 - Reed Smith9 1NF:

A337 - Reed Smith10 What is wrong with this solution? Partial Dependencies ◦ For example, the sales order number is not relevant in the determination of the item name ◦ Similarly, the customer code and customer name do not depend upon the Item ID, they only depend upon the sales order number.

Second Normal Form Eliminate Partial Dependencies ◦ Write each key component on separate line, and then write the original (composite) key on the last line ◦ Each component will become the key in a new table Identify the Dependent Attributes ◦ Determine which attributes are dependent on which other attributes

Creating 3 Tables SO_Number, Item_Number SO_Number, Item_Number

A337 - Reed Smith13 2NF:

A337 - Reed Smith14 What is wrong with this solution? Transitive Dependencies ◦ Notice that the third column of the Sales Orders file has the Customer name and that depends upon the customer number. ◦ But the customer number is not the primary key

Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel Third Normal Form 1. For every transitive dependency, write its determinant as a PK for a new table 2. Identify the attributes dependent on each determinant identified in Step 1 and identify the dependency 3. Remove the dependent attributes in transitive relationship(s) from each table that has such a transitive relationship

A337 - Reed Smith16 3NF:

Denormalization Creation of normalized relations is important database design goal Processing requirements should also be a goal If tables decomposed to conform to normalization requirements ◦ Number of database tables expands 17

Denormalization ( continued ) Joining larger number of tables takes additional disk input/output (I/O) operations and processing logic ◦ Reduces system speed Conflicts among design efficiency, information requirements, and processing speed are often resolved through compromises that may include denormalization A337 Database Systems: Design, Implementation, & Management, 6 th Edition, Rob & Coronel 18