Page 280 - Information_Practice_Fliipbook_Class11
P. 280
• Backup and Recovery: In case of some user error, hardware failure, or any catastrophic event, the DBMS
can recover the database from the point it was failed.
• Integrity: DBMS ensures the correctness of data stored by introducing various integrity constraints. For
example, DBMS will ensure that no two employees have the same Aadhaar number.
Ø Disadvantages of DBMS:
• Initial Cost: Applications based on DBMS involve higher costs of hardware and DBMS software.
• Complexity: As DBMS offers various functionalities, it becomes a large and complex software, requiring
training of application developers.
Ø Data Model: A data model is a representation of the structure of real-world objects. A data model comprises
the following components:
• Entity: An entity is a real-world object (such as employee and department in a company) about which
data is stored in the database.
• Attribute: An attribute defines the characteristics of an entity. For example, employee id, date of birth,
address, and salary form attributes of an employee.
• Relationship: A relationship specifies how two entities are related to each other. For example, in a ccompany's
database the employee entity is related to the department entity since an employee works in a department.
• Constraint: A constraint describes the restrictions imposed on the data stored. For example, each
employee should have a unique Aadhaar number.
Ø Relational Data Model: A relational database management system (RDBMS) stores data in the from of
relations, also called tables. A table has the following characteristics:
• Each column of a table represents a unique attribute.
◦ An attribute has a type denoting the type of data in the column.
◦ Value in a column must conform to the attribute type.
• Relational schema describes the structure of a relation. It includes the names of attributes of a table,
types of attributes, and the constraints on a table.
• Degree: No of attributes in the relation is called degree of the relation.
• Tuple: A row in a table is called a tuple.
• Entity Set: The set of all tuples at a specific time in a relation is known as the entity set.
• Relation state, Relation instance: Other names for the entity set.
• Cardinality: Numbers of tuples in the entity set.
• Order within a table:
◦ Rows in a table are considered unordered.
◦ Columns in a table are considered unordered.
Ø NULL: When value of an attribute is unknown, it is denoted by NULL.
Ø Candidate Key: The minimal set of attributes that identifies a tuple in a table is called its candidate key.
Minimality means that no attribute is superfluous.
Ø Primary Key: One of the candidate keys is chosen as primary key.
Ø Entity Integrity Constraint: No primary key attribute can have a NULL value
Ø Alternate Key: A candidate keys that is not chosen as primary key.
Ø Foreign Key: A non-key attribute of table R1 that is part of primary key of table R2 is a foreign key of table R1.
Ø Foreign Key Constraint: If a table R1 has a foreign key (say, A)which is part of the primary key of another
table R2, then
• the value of foreign key A maybe be NULL.
• the value of foreign key A in table R1 maybe be non- NULL (say, val). In this case. there must be a
tuple in table R2 having val as the value of the referred attribute.
266 Touchpad Informatics Practices-XI

