Lesson 1 - Conceptual Models

A Logical View of Data

- Placing the DBMS between the application and the database eliminates most of the file

system’s inherent limitations.

- The relational model enables you to view data logically rather than physically.

Tables and Their Characteristics

- The logical view of the relational database is facilitated by the creation of data

relationships based on a logical construct known as a relation.

- A is perceived as a two-dimensional structure com table posed of rows and columns, and

is called a . relation

- As far as the table’s user is concerned, a table contains a group of related entity

occurrences, or an entity set.

- The characteristics of a relational table are:

1. A table is perceived as a two dimensional structure composed of rows and

columns.

2. Each table row ( ) represents a single entity occu tuple rrence within the entity set.

3. Each table column represents an attribute, and each column has a distinct name.

4. Each intersection of a row and column represents a single data value.

5. All values in a column must conform to the same data format.

6. Each column has a specific range of values known as the attribute . domain

7. The order of the rows and columns is immaterial to the DBMS.

8. Each table must have an attribute or combination of attributes that uniquely

identifies each row.

Keys

- A consists of one or more attributes that determi key ne other attributes.

- Determination is that state in which knowing the value of one attribute makes it possible

to determine the value of another.

- Functional dependence means that the value of one or more attributes determines the

value of one or more other attributes.

- The attribute whose value determines another is called the . determinant

- The attribute whose value is determined by the other attribute is called the . dependent

- Full functional dependence is used to refer to functional dependencies in which the

entire collection of attributes in the determinant is necessary for the relationship.

Types of Keys

- A is a key composed of more than one attribute. composite key

- An attribute that is part of a key is called a . key attribute

- A is a key that can uniquely identify any row in superkey the table — in other words, it

functionally determines every attribute in the row.

- A is a minimal superkey without any unnecessary a candidate key ttributes, based on a

full functional dependency.

- Entity integrity is the condition in which each row in the table has its own unique

identity.

- All of the values in the primary key must be unique

- No key attribute in the primary key can contain a null.

- A is the primary key of one table that has been p foreign key (FK) laced into another

table to create a common attribute.

- Referential integrity is the condition in which every reference to an entity instance by

another entity instance is valid.

- A is defined as a key that is used strictly for d secondary key ata retrieval purposes.

Relational Algebra

- Relational algebra defines the theoretical way of manipulating table contents using

relational operators.

Formal Definitions and Terminology

- A is a variable that holds a relation. The table relvar structure is properly called a relvar,

and the data in the structure would be a relation. The relvar is a container (variable) for

holding relation data, not the relation itself.

- A relvar has two parts:

- the heading contains the names of the attributes

- the body contains the relation

Relational Set Operators

- The relational operators have the property of ; th closure e use of relational algebra

operators on existing relations produces new relations.

- SELECT (Restrict):

- SELECT, also known as RESTRICT, is referred to as a unary operator because it

only uses one table as input.

-

- PROJECT

- PROJECT yields all values for selected attributes. It is also a unary operator,

accepting only one table as input.

-

- UNION

- UNION combines all rows from two tables, excluding duplicate rows. To be used

in the UNION, the tables must have the same attribute characteristics. The

columns and domains must be compatible, when they are said to be

union-compatible.

-

- INTERSECT

- INTERSECT yields only the rows that appear in both tables.

- The tables must be union-compatible to yield valid results.

-

- DIFFERENCE

- DIFFERENCE yields all rows in one table that are not found in the other table;

that is, it subtracts one table from the other.

- The tables must be union-compatible to yield valid results.

-

- PRODUCT

- PRODUCT yields all possible pairs of rows from two tables.

-

- JOIN

- JOIN allows information to be intelligently combined from two or more tables.

JOIN is the real power behind the relational database, allowing the use of

independent tables linked by common attributes.

- The CUSTOMER and AGENT tables will be used to illustrate several types of

joins.,

-

- A links tables by selecting only the rows with co natural join mmon values in their

common attributes

- First, a PRODUCT of the tables is created.

- Second, a SELECT is performed to yield only the rows for which the

AGENT_CODE values are equal

-

- A PROJECT is performed to yield a single copy of each attribute,

eliminating duplicate columns.

-

- The final outcome of a natural join yields a table that does not include

unmatched pairs and provides only the copies of the matches.

- If no match is made between the table rows, the new table does not

include the unmatched row.

- The column on which the join was made, occurs only once in the new

table.

- If the same AGENT_CODE were to occur several times in the AGENT

table, a customer would be listed for each match.