Concept of rdbms Tutorial Study Material Notes with Examples

Relational Database Management System (RDBMS)

A System in who users access data with use of relation (i.e., data must be available   in tabular form i.e., as a collection of tables, where each consisting a set of rows and columns). That provides a variety of relational  operator so that we can manipulate the data in tabular form.

Features of RDBMS

There are many features of RDBMS

• We can create multiple relations (tables) and feed data into them.
• Provides an interactive query language.
• We can retrieve information from more than one table using join concept_.
• Provides a catalog or dictionary which consists of system tables.

RDBMS Vocabulary

Relation A table.

Attribute A column in a  table

Degree Number of attributes in a relation.

Cardinality Number of  tuples in table .

Domain  or Type A pool NULL Special value form which specific attributes draw their values

NULL  Special value for unknown’ or ‘undefined’

Relational Data Model It provides mechanisms (languages) for defining data structures operations for retrieval and modification of data and integrity constraints.

Keys

An attribute or set of attributes whose values uniquely identify each entity in an entity set is called a key for that entity set.  Different types of keys  are as follows

Super Key

It is a set of one or no e attributes that allow us to identify uniquely an entity in the entity set. e.g.,  the Custoomer_id  attribute of the entity set, customer is  sufficient to distinguish one customer entity from another. Thus Customer_id is a super key. Similarly, the combination of Customer_Name

And customer_Id  is a super key for the entity set customer.  Customer_Name  attribute of customer is not a super key, because  people right have the same name.

Candidate Key

Since as we saw a super key may contain extraneous attributes (i.e.,  Customer_Name here in the above case). If K is a super key, then candidate key is any superset of K. We are often interested in super keys _for which no proper subset is a super key. Such minimal super keys are _e candidate keys. Suppose that a combination of Customer_Name and Customer_Street is sufficient to distinguish among members of the also Homer entity set. Then, both {Customer_Id} and {Customer_Name, Customer_Street} are candidate keys. Although, the attributes customer_ld and Customer_Name together can distinguish customer entities, their combination does not form a candidate key, since the attribute Customer_Id alone is a candidate key.

Primary Key

A candidate key that is chosen by the database designer as the principal  means of identifying entities within an entity set. A key (primary, candidate  and super) is a property of the entity set rather than of the individual entities.

Alternate Key

A table may have one or more choices for the primary key. Collectively these are known as candidate keys as discuss earlier. One is selected as the primary key. Those not selected are known as secondary keys or alternative keys.

Secondary Key

An attribute or set of attributes that may not be a candidate key but that  classifies the entity set on a particular characteristics. e.g., suppose entity Set employee having the attribute department whose value identifies all Instances employee who belong to  a given department.

Foreign Key

Foreign key is generally a primary key from one table that appears as a field in another, where the first table has a relationship to the second. In other TBL1  with a primary key. A that linked to a table TBL2  where A was a field in TBL2, then A would be a foreign key in TBL2

Simple and Composite Key

Any key consisting of a single attribute is called a simple key, key,  while that consisting of  a combination of attributes is called a composite key.

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