Database Design 2

7/28/2019 Database Design 2

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Database Design 2: Design Method

Introduction Two-step process for database design Information-level design: completed independentlyof any particular DBMS

Physical-level design: information-level design adapted for the specific DBMSthat will be used

Must consider characteristics of the particular DBMS

User Views User view:set of requirements necessary to support operations of a particular

database user Cumulative design:supports all user views encountered during design process

Information-Level Design Method For each user view:

1. Represent the user view as a collection of tables2. Normalize these tables3. Identify all keys in these tables4. Merge the result of Steps 1 through 3 into the cumulative design

Represent the User View As a Collection of Tables Step 1: Determine the entities involved and create a separate table for each type

of entity Step 2: Determine the primary key for each table Step 3: Determine the properties for each entity Step 4: Determine relationships between the entities

One-to-many

Many-to-many One-to-one

One-to-many relationship: include primary key of the one table as a foreign keyin the many table

Many-to-many relationship: create a new table whose primary key is thecombination of the primary keys of the original tables

One-to-one relationship: simplest implementation is to treat it as a one-to-manyrelationship

Normalize the Tables Normalize each table Target is third normal form

Careful planning in early phases of the process usually rules out need toconsider fourth normal form

Identify All Keys For each table, identify:

Primary key Alternate keys Secondary keys Foreign keys

Alternate key: column(s) that could have been chosen as a primary key but wasnot Secondary keys:columns of interest strictly for retrieval Foreign key: column(s) in one table that is required to match value of the primary

key for some row in another table or is required to be null Used to create relationships between tables Used to enforce certain types of integrity constraints


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Types of Primary Keys Natural key:consists of a column that uniquely identifies an entity

Also called a logical key or an intelligent key Artificial key: column created for an entity to serve solely as the primary key and

that is visible to users

Surrogate key: system-generated; usually hidden from users Also called a synthetic key

Database Design Language (DBDL)

Table name followed by columns in parentheses Primary key column(s) underlined

AK identifies alternate keys SK identifies secondary keys FK identifies foreign keys

Foreign keys followed by an arrow pointing to the table identified by the

foreign key

DBDL for the Employee table


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NORMALIZATION

First Normal Form Repeating group: multiple entries for a single record Unnormalized relation: contains a repeating group

Table (relation) in first normal form (1NF) does not contain repeating groups

Orders (OrderNum, OrderDate, (PartNum, NumOrdered) )

Sample unnormalized table

Orders (OrderNum, OrderDate, PartNum, NumOrdered)

Result of normalization (conversion to first normal form)


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Second Normal Form

Sample Orders table

Orders (OrderNum, OrderDate, PartNum, Description, NumOrdered,QuotedPrice)

Functional dependencies:OrderNum OrderDatePartNum DescriptionOrderNum, PartNum NumOrdered, QuotedPrice, OrderDate, Description

Update anomalies Update Inconsistent data Additions Deletions

Nonkey column (nonkey attribute): not part of primary key

Table (relation) in second normal form (2NF) Table is in first normal form No nonkey column is dependent on only a portion of primary key

Dependency diagram: arrows indicate all functional dependencies Arrows above boxes: normal dependencies Arrows below boxes: partial dependencies

Partial dependencies:only on a portion of the primary key

Dependences in the Orders table


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Conversion to second normal form

Third Normal Form Customer (CustomerNum, CustomerName, Balance, CreditLimit, RepNum,

LastName, FirstName) Functional dependencies:

CustomerNum CustomerName, Balance, CreditLimit, RepNum,LastName, FirstName

RepNum LastName, FirstName


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Sample Customer table

2NF tables may still contain problems Redundancy and wasted space Update anomalies

Update Inconsistent data Additions Deletions

Determinant: column(s) that determines another column

Table (relation) in third normal form (3NF) It is in second normal form Its only determinants are candidate keys

Dependencies in the Customer table

Correction procedure For each determinant that is not a candidate key, remove from table the

columns that depend on this determinant Create new table containing all columns from the original table that

depend on this determinant Make determinant the primary key of new table


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Conversion to third normal form

Database Design 2

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