Relational Model or Relational Database – (Part 1)
The relational model is a lower level model. It is based on the concept of a relation, which is physically represented as a table. A table is a collection of rows & columns . The relational model uses a collection of tables to represent both data and the relationships among those data.
The tables are used to hold information about the objects to be represented in the database. A relation or a table is represented as a two dimensional form in which the rows of the table corresponds to individual records and the columns corresponds to attributes.
Each row is called a tuple and each column is called an attribute.For example, a student relation is represented by the STUDENT table having columns for attributes SID, NAME and BRANCH.
SID : Key Number of Records = Cardinality Number of Fields = Arity Student (SID,Name,Branch) = Relational Schema (Table Abstraction)
The SID here is the primary key as it identifies a student record or tuple uniquely.(A primary key is the key applied on an attribute(SID) which recognize a tuple.
The Cardinality of the Relation or table is defined as the number of records in the STUDENT relation which is 4.
The Arity is defined as the number of fields or columns in the relation.
Domain of an Attribute –
Domain of an attribute is the set of allowable values for that attribute. It is a pool of values from which the actual values appearing in a given column are drawn. For example, the values appearing in the SID column are drawn from the domain of all SID. Domains may be distinct, or two or more attributes may have same domain.
Operations in Relational Model –
- Insertion – A new student record can be easily inserted in the table.
- Deletion – An existing student record or tuple can easily be deleted from the STUDENT relation.
- Updation – An existing student record can be update easily. For example, if a student S2 changes its BRANCH from CS to IT, then it can easily be changed
Advantages of Relational Model –
- Easy to use an understand
- Very flexible.
- Widely used.
- Provides excellent support for adhoc queries.
- Users need not consider issues such as storage structure and access strategy.
- Specify control and authorization can be implemented more easily.
- Data independence is achieved more easily with normalisation structure used in a relational database.
Disadvantages of Relational Model –
- For large databases, the performance in responding to queries is definitely degraded.
- The processing requirements need to construct the indexes. So, the index position of the file must be created and maintained along with the file records themselves.
- The file index must be searched sequentially before the actual file records are obtained. This wastes time.
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[Relational Model (Part2)]