C170 Lecture Notes - Lecture 1: Functional Dependency, Relational Model, Railways Act 1921

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26 Sep 2019
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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 table is perceived as a two-dimensional structure composed 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 (tuple) represents a single entity occurrence 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 key consists of one or more attributes that determine 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.
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Types of Keys
- A composite key is a key composed of more than one attribute.
- An attribute that is part of a key is called a key attribute.
- A superkey is a key that can uniquely identify any row in the table — in other words, it
functionally determines every attribute in the row.
- A candidate key is a minimal superkey without any unnecessary attributes, 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 foreign key (FK) is the primary key of one table that has been placed 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 secondary key is defined as a key that is used strictly for data retrieval purposes.
Relational Algebra
- Relational algebra defines the theoretical way of manipulating table contents using
relational operators.
Formal Definitions and Terminology
- A relvar is a variable that holds a relation. The table 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
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Relational Set Operators
- The relational operators have the property of closure; the 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
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