RDBMS stands for relational database management system. The RDBMS is a collection of programs that are used to create and maintain relational databases. A database in which the data is stored in relations is called relational database. The most common type of DBMS is the relational database that is used on all the systems from micro computers to the mainframes computer.
The RDBMS software is used to translate the user’s requests into commands. The requested data is retrieved and located by these commands. Data dictionary is one of the important concepts of RDBMS.
The data dictionary stores information about the data. It also stores the description of the structure of the relation in the database and also stores the description of data relationships and the integrity constraints on data.
Facilities provided by RDBMS
A good RDBMS provides the following facilities:
- Data security
- Data integrity
- Easy access
- Backup and recovery
Relation is another term that is used for table. In relational database the data is held in tables also called relations and these tables are linked by means of common fields. A table in a database has a unique name that identifies its contents. One row of the table holds one record and each column in the table holds one field or one attribute.
A table holding data about an entity BOOK may have the following rows and columns:
|Book ID||Title||Author||Date Published|
|30223||Introduction to computers||Abraham||1998|
|49923||Mastering Access 2000||Tim||2000|
There is a standard notation for describing a table in a relational database. For example, to describe the table shown above you would write
BOOK(Book ID, Title, Author, Date Published)
Note that the entity name is shown in uppercase letter, the primary key field (unique identifier) is underlined and the attributes are shown in brackets, separated by commas.
Parts of a Table
A table consists of following parts:
- Field or Attribute
- Record or Row
Field or Attribute
A field is a combination of related characters. A field is known as attribute in relational database. An attribute is a named column of a relation. Each field in a table has unique name and predefined data type. Each field may contain certain default value. Following are three examples of fields:
Record or Row
A record is a collection of related field. A record is known as tuple in relational database. Record is also called row. In the above figure, the information of each student is stored in a separate row.
Each record in the table stores data about different object. Following are two examples of records in a table:
A database table can have indexes on as many fields as you choose. An index is a list of numerical values which gives the order of the records when they are sorted on a particular field. The DBMS constructs and maintains all the indexes automatically. There are advantages and disadvantages of having multiple indexes:
- In large tables they speed up queries considerably
- When a report is required in the sequence of the indexed field, they avoid having to sort the database
- On the negative side, they slow down data entry and editing, because the indexes have to be updated each time a record is added or deleted.
The capability to ensure referential integrity is an important feature of Access. Referential integrity enforcement prevents you from deleting or modifying values of a primary table’s record on which related records depend. If you try to delete a student’s record from students table, Access prevents you from doing so.
Achieving Referential Integrity
The referential integrity can be achieved by connecting two relations by specifying relationships between them. When two relations are connected, one relation is called parent relation and the other is called dependent relation.
The parent relation contains the primary key and the dependent relation contains foreign key. A value entered in the foreign key attribute in dependent relation must exist in the primary key attribute of parent table.
The record is not stored in the database if the user violates of this rule. In this way, the referential integrity is achieved. It ensures that the data in both relations is consistent.
The entity integrity is a constraint on primary key value. It states that any attribute of a primary key cannot contain a null value. If primary key contains a null value, it is not possible to uniquely identify a record in a relation.
Attaining Entity Integrity
The entity integrity can be attained by specifying primary key in a relation. When a primary key constraint is specified on a relation, the DBMS automatically applies the entity integrity on the attribute that is used as primary key.
The attribute cannot contain null value in it. DBMS displays as error message if the user attempts to save a record without specifying a valid value in the primary key attribute. If the user specifies composite primary key, the entity integrity is also applied to all attributes that are part of composite primary key.