Keys in DBMS

By Priyanshu Vaish|Updated : September 30th, 2022

The keys in DBMS ensure that each record in a table is precisely identified. As you already know, a table represents a singular collection of similar objects or events. Keys are a major component of table-level integrity and relationship-level integrity.

For instance, they enable you to ensure that a table has unique records and that the fields you use to establish a relationship between a pair of tables always contain matching values. Read ahead to learn more about the keys in DBMS along with various types of keys in DBMS and their examples.

Table of Content

What are the Keys in DBMS?

Databases are used to store massive amounts of data spread across multiple tables. Each table may contain thousands of rows. Needless to say, there will be various duplicate rows with redundant data. The Keys in DBMS help to store the data uniquely.

Keys in DBMS Definition

The keys in DBMS refer to an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation). A key is also used when we want to establish relationships between the different columns and tables of a relational database.

The individual values present in a key are commonly referred to as key values. This article will look at the types of Keys in DBMS according to the Gate Syllabus for CSE.

Role of Keys in DBMS

Keys in DBMS allow the sharing of database details among many apps or people in order to provide drivers with a single all-encompassing details repository. The keys in DBMS act as a bridge between the user and the database. A database management system (DBMS) provides roles such as:

  • Improved data sharing
  • Improved data security
  • Better data integration
  • Improved data access
  • Increased user productivity

Why are keys required in DBMS?

The keys in DBMS are used in the definitions of various types of integrity constraints. A table in a data source is a collection of events or records for a specific relationship. Today, there could be thousands and thousands of such documents, many of which are duplicates.

There must be a strategy in place to determine each record uniquely and separately, ensuring that there are no duplicates. Keys in DBMS allow us to be completely free of this particular hassle. Keys are an important component of both table-level and relationship-level integrity.

8 Types of Keys in DBMS

There are 8 types of keys in DBMS, which are listed below:

  • Candidate Key
  • Super Key
  • Primary Key
  • Alternate Key
  • Foreign Key
  • Simple Candidate Key
  • Compound Candidate Key
  • Overlapping Candidate Key

Let's check out these keys in DBMS with examples one by one.

Candidate Keys in DBMS

The minimum set of attributes that can uniquely identify each and every tuple of the relation. If a relation schema has more than one key, each is called a candidate keys in DBMS.

Super Keys in DBMS

The set of attributes that can differentiate records of a relation uniquely. It may not be a minimal attribute set. The candidate keys in DBMS are always the super key, but vice versa is not true.

A super key of the relation schema R= {A1, A2,..., An) is a set of attributes S ⊆ R with the property that no two tuples, t1 and t2, in any legal relation state r of R will have t1[S] = t2[S]. A key K is a super key with the additional property that removing any attribute from K will cause K not to be a super key anymore.
The difference between a key and a super key is that a key has to be minimal; that is, if we have a key K= {A1, A2,..., An) of R, then K-{Aj} is not a key of R for any Aj 1 ≤ I ≤ k.

Primary Keys in DBMS

A primary key is a single attribute or combination of attributes that can uniquely identify a row of data in each table, among various candidate keys in DBMS, at least one candidate key whose field value is not null.

Alternate Keys in DBMS

All candidate keys in DBMS of relational schema except primary keys are the alternate keys. The alternate keys are also called secondary keys in DBMS. Alternate keys(secondary keys) allowed NULL values.

Foreign Keys in DBMS

A foreign key is the set of attributes in a table used to refer to the primary key or alternative key of the same or another table. The foreign key allows null values in the column.

Whenever the same type of attribute exists in two different tables, the attribute in one of the tables is declared the primary key. On the other, it is made a foreign key so that the values in both become consistent. A foreign key is dependent on the primary key. Therefore, it is defined over two tables:

  • Referenced relation: The relation to which other relations refer is called a referenced relation.
  • Referencing relation: The relation which is referencing another relation is called referencing relation.

Check out the difference between the primary key and foreign key in DBMS to understand the keys better.

Simple Candidate Keys in DBMS

Only one attribute forms a candidate key. Such a key is called a simple candidate key.

Compound Candidate Keys in DBMS

A candidate key with more than one attribute is called a compound candidate key.

Overlapping Candidate Keys in DBMS

The candidate key which has at least one common attribute between multiple candidate keys is known as the overlapping candidate key.

Advantages of Keys in DBMS

Keys in DBMS were released to address the fundamental issues associated with storing, securing, accessing, managing, and auditing data in standard file systems. By utilizing keys in DBMS, software users and organizations can obtain the following benefits:

  • Improved Information Security
  • Simple Information Sharing
  • Data Integration
  • Abstraction & Independence
  • Uniform Management and Monitoring
  • DBMSs are essential

Important GATE Topics

OR Gate Truth TableRtc Full Form
Universal GatesFCFS Scheduling Full Form
Multistage AmplifierTypes Of Loads
Introduction To C ProgrammingE-Commerce Mcq
Efficiency Of An AlgorithmLaser Full Form

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FAQs on Keys in DBMS

  • A key refers to an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation). A key is also used when we want to establish relationships between the different columns and tables of a relational database.

  • Keys in DBMS play an important role in the relational database. It is used to uniquely identify any record or row of data from the table. It is also used to establish and identify relationships between tables.

  • Alternate keys are those candidate keys that are not the primary key. There can be only one primary key for a table. Therefore all the remaining Candidate keys are known as Alternate or Secondary keys.

  • A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table.

  • Both primary and unique keys in DBMS are used to define a row in a table uniquely. Primary Key creates a clustered column index, whereas a unique creates an Unclustered column index. A primary key doesn't allow NULL value. However, a unique Key does allow one NULL value.

  • The different types of keys in DBMS are as follows:

    • Candidate Key
    • Super Key
    • Primary Key
    • Alternate Key
    • Foreign Key
    • Simple Candidate Key
    • Compound Candidate Key
    • Overlapping Candidate Key


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