Relational Model
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The document summarizes key concepts of the relational database model including: 1. The relational model uses tables to represent data and relationships, with each table having columns and rows. 2. Key characteristics are that it is the primary commercial data model, provides a simple way to represent data, and uses a record-based structure with fixed-format records and fields. 3. Relational databases have a schema defining relations (tables) and attributes (columns), with each relation made up of tuples (rows) that contain values from the defined domains.
![In the account relation of Figure 1, there are seven tuples. Let the tuple variable t refer to the first tuple of
the relation.We use the notation t[account-number] to denote the value of t on the account-number attribute.
Thus, t[account-number] = “A-101,” and t[branch-name] = “Downtown”. Alternatively, we may write t[1] to
denote the value of tuple t on the first attribute (account-number), t[2] to denote branch-name, and so on.
Since a relation is a set of tuples, we use the mathematical notation of t ∈ r to denote that tuple t is in
relation r.
2.2 Database Schema
Database schema: Logical design of the database.
Database instance: A snapshot of the data in the database at a given instant in time.
Relation schema:
The concept of relation schema corresponds to the programming-language notion of type definition.
There should be a name of a relation schema. The convention is to use names beginning with an
uppercase letter.
In general, a relation schema consists of a list of attributes and their corresponding domains.
Branch_schema = (branch_name, branch_city, assets)
Customer_schema = (customer_id, customer_name, customer_street, customer_city)
Loan_schema = (loan_no., branch_name, amount)
Relation instance:
The concept of relation instance corresponds to the programming-language notion of a value of a
variable. In other words, A relation instance is a tuple or row in a relation, i.e. one particular combination
of attribute values.
Let there be a relation Student. The schema of the relation Student has the following attributes:
{ID,first_name,last_name}
Now each of the attributes have their own domain i.e. a range of values they can take up.
Student
ID f_name l_name
1 Bob Dylan
2 Adam Levine
3 Chris Martin
(1, Bob, Dylan) is a relation instance.
Table 1. Terms and their synonyms by A. Watt.](https://image.slidesharecdn.com/lecture-06-200106055302/85/Relational-Model-2-320.jpg)
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Relational Model
- 1. 1. Relational Model The relational model uses a collection of tables to represent both data and relationships among those data. Each table has multiple columns and each column has a unique name. Characteristics 1. The primary data model for commercial data-processing applications. 2. It provides very simple but powerful way of representing data that eases the job of the programmer. 3. This is a record-based model. The database is structured in fixed-format records of several types. Each table contains records of a particular type. Each record type defines a fixed no. of fields, or attributes. The columns of the table correspond to the attributes of the record type. 4. The relational model is at a lower level than the E-R model. Database designs are often carried out in the E-R model and then translated to the relational model. 2. Structure of Relational Databases 2.1 Basic Structure Relation and tuple: In relational model these are synonymous to table and row. Attribute: In relational model we refer column headers as attributes. Domain: a domain is a set of acceptable values that a column is allowed to contain. This is based on various properties and the data type for the column. For the attribute branch_name, the domain is the set of all branch names. Atomic domain: A domain is atomic if elements of the domain are considered to be indivisible parts. For example: The domain of Marital Status has a set of possibilities: Married, Single, Divorced. Null value: One domain value that is a member of any possible domain is the null value, which signifies that the value is unknown or does not exist. Example: telephone_no. Null values cause a number of difficulties while accessing or updating the database, thus should be eliminated if at all possible. Tuple variable: A tuple variable is variable that takes on tuples of a particular relation schema as values. That is, every value assigned to a given tuple variable has the same number and type of fields. Figure 1. The account relation
- 2. In the account relation of Figure 1, there are seven tuples. Let the tuple variable t refer to the first tuple of the relation.We use the notation t[account-number] to denote the value of t on the account-number attribute. Thus, t[account-number] = “A-101,” and t[branch-name] = “Downtown”. Alternatively, we may write t[1] to denote the value of tuple t on the first attribute (account-number), t[2] to denote branch-name, and so on. Since a relation is a set of tuples, we use the mathematical notation of t ∈ r to denote that tuple t is in relation r. 2.2 Database Schema Database schema: Logical design of the database. Database instance: A snapshot of the data in the database at a given instant in time. Relation schema: The concept of relation schema corresponds to the programming-language notion of type definition. There should be a name of a relation schema. The convention is to use names beginning with an uppercase letter. In general, a relation schema consists of a list of attributes and their corresponding domains. Branch_schema = (branch_name, branch_city, assets) Customer_schema = (customer_id, customer_name, customer_street, customer_city) Loan_schema = (loan_no., branch_name, amount) Relation instance: The concept of relation instance corresponds to the programming-language notion of a value of a variable. In other words, A relation instance is a tuple or row in a relation, i.e. one particular combination of attribute values. Let there be a relation Student. The schema of the relation Student has the following attributes: {ID,first_name,last_name} Now each of the attributes have their own domain i.e. a range of values they can take up. Student ID f_name l_name 1 Bob Dylan 2 Adam Levine 3 Chris Martin (1, Bob, Dylan) is a relation instance. Table 1. Terms and their synonyms by A. Watt.
- 3. A. S. M. Shafi Lecturer Department of Computer Science and Engineering Khwaja Yunus Ali University Enaytpur, Sirajgonj-6751, Bangladesh