Information Management - 3 Flashcards
Relational model
views data logically rather than physically
Table
- Structural and data independence
- resembles file conceptually
Relational database
much easier to understand rather than hierarical and network model
Tables and Their Characteristics
- Logical view if relational database based on relation
- Table is a two-dimensional structure composed of row and columns
- Contains group of related entities
Characteristics of Relational Table
- table is a two-dimensional structure composed of rows and columns
- each table row must represent a single entity
- each table column must represent an attribute
- all values in column must conform a single format
- Each row/column intersection represents a data
- each column has a specific range of values kniwn attribute domain
- order of rows and columns are immaterial on DBMS
- each table must have an attribute that uniquely identifies each row
Key
- one or more attribute determines another attribute
- role is based on determination
-If you know the value of attribute A, you can determine the value of attribute B - **Functional dependence - Attribute B is functionally dependent on A if all rows in table that agree in value for A also agree in value for B
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Composite Key
composed of one or more attribute
Key Attribute
an attribute that is a part of a key
Superkey
Any key that uniquely identifies each row
Candidate Key
superkey without unnecessary attributes
Null
-no data entry
-may composed of:
* missing value
* known but not attribute value
* not applicable condition
-can produce COUNT, SUM, AVERAGE
-create logical problem
Controlled redundancy
- makes relational database work
- tables within database share common value
- multiple occurance of values not redundant when required
- Redundancy exists only when there is unnecessary duplication of attribute values
Foreign Key
attribute whose values match primary key
Referential Integrity
FK that contains value thatb refers to an existing table row
Secondary Key
key used strictly for retrieval purposes