DBMS Keys: Candidate, Super, Primary, Foreign (Example)

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What are Keys in DBMS?

KEYS in DBMS is an attribute or set of attributes which helps you to identify a row(tuple) in a relation(table). They allow you to find the relation between two tables. Keys help you uniquely identify a row in a table by a combination of one or more columns in that table. Key is also helpful for finding unique record or row from the table. Database key is also helpful for finding unique record or row from the table.

Example:

Employee IDFirstNameLastName
11AndrewJohnson
22TomWood
33AlexHale

In the above-given example, employee ID is a primary key because it uniquely identifies an employee record. In this table, no other employee can have the same employee ID.

Why we need a Key?

Here are some reasons for using sql key in the DBMS system.

  • Keys help you to identify any row of data in a table. In a real-world application, a table could contain thousands of records. Moreover, the records could be duplicated. Keys ensure that you can uniquely identify a table record despite these challenges.
  • Allows you to establish a relationship between and identify the relation between tables
  • Help you to enforce identity and integrity in the relationship.

Types of Keys in Database Management System

There are mainly seven different types of Keys in DBMS and each key has it’s different functionality:

  • Super Key –  A super key is a group of single or multiple keys which identifies rows in a table.
  • Primary Key –  is a column or group of columns in a table that uniquely identify every row in that table.
  • Candidate Key –  is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes.
  • Alternate Key –  is a column or group of columns in a table that uniquely identify every row in that table.
  • Foreign Key –  is a column that creates a relationship between two tables. The purpose of Foreign keys is to maintain data integrity and allow navigation between two different instances of an entity.
  • Compound Key –  has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database.
  • Composite Key –  An artificial key which aims to uniquely identify each record is called a surrogate key. These kind of key are unique because they are created when you don’t have any natural primary key.
  • Surrogate Key –  An artificial key which aims to uniquely identify each record is called a surrogate key. These kind of key are unique because they are created when you don’t have any natural primary key.

What is the Super key?

A superkey is a group of single or multiple keys which identifies rows in a table. A Super key may have additional attributes that are not needed for unique identification.

Example:

EmpSSNEmpNumEmpname
9812345098AB05Shown
9876512345AB06Roslyn
199937890AB07James

In the above-given example, EmpSSN and EmpNum name are superkeys.

What is a Primary Key?

PRIMARY KEY is a column or group of columns in a table that uniquely identify every row in that table. The Primary Key can’t be a duplicate meaning the same value can’t appear more than once in the table. A table cannot have more than one primary key.

Rules for defining Primary key:

  • Two rows can’t have the same primary key value
  • It must for every row to have a primary key value.
  • The primary key field cannot be null.
  • The value in a primary key column can never be modified or updated if any foreign key refers to that primary key.

Example:

In the following example, <code>StudID</code> is a Primary Key.

StudIDRoll NoFirst NameLastNameEmail
111TomPriceabc@gmail.com
212NickWrightxyz@gmail.com
313DanaNatanmno@yahoo.com

What is the Alternate key?

ALTERNATE KEYS is a column or group of columns in a table that uniquely identify every row in that table. A table can have multiple choices for a primary key but only one can be set as the primary key. All the keys which are not primary key are called an Alternate Key.

Example:

In this table, StudID, Roll No, Email are qualified to become a primary key. But since StudID is the primary key, Roll No, Email becomes the alternative key.

StudIDRoll NoFirst NameLastNameEmail
111TomPriceabc@gmail.com
212NickWrightxyz@gmail.com
313DanaNatanmno@yahoo.com

What is a Candidate Key?

CANDIDATE KEY is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes. The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key. A table can have multiple candidate keys but only a single primary key.

Properties of Candidate key:

  • It must contain unique values
  • Candidate key may have multiple attributes
  • Must not contain null values
  • It should contain minimum fields to ensure uniqueness
  • Uniquely identify each record in a table

Example: In the given table Stud ID, Roll No, and email are candidate keys which help us to uniquely identify the student record in the table.

StudIDRoll NoFirst NameLastNameEmail
111TomPriceabc@gmail.com
212NickWrightxyz@gmail.com
313DanaNatanmno@yahoo.com

What is the Foreign key?

FOREIGN KEY is a column that creates a relationship between two tables. The purpose of Foreign keys is to maintain data integrity and allow navigation between two different instances of an entity. It acts as a cross-reference between two tables as it references the primary key of another table.

Example:

DeptCodeDeptName
001Science
002English
005Computer
Teacher IDFnameLname
B002DavidWarner
B017SaraJoseph
B009MikeBrunton

In this key in dbms example, we have two table, teach and department in a school. However, there is no way to see which search work in which department.

In this table, adding the foreign key in Deptcode to the Teacher name, we can create a relationship between the two tables.

Teacher IDDeptCodeFnameLname
B002002DavidWarner
B017002SaraJoseph
B009001MikeBrunton

This concept is also known as Referential Integrity.

What is the Compound key?

COMPOUND KEY has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database. However, when combined with the other column or columns the combination of composite keys become unique. The purpose of the compound key in database is to uniquely identify each record in the table.

Example:

OrderNoPorductIDProduct NameQuantity
B005JAP102459Mouse5
B005DKT321573USB10
B005OMG446789LCD Monitor20
B004DKT321573USB15
B002OMG446789Laser Printer3

In this example, OrderNo and ProductID can’t be a primary key as it does not uniquely identify a record. However, a compound key of Order ID and Product ID could be used as it uniquely identified each record.

What is the Composite key?

COMPOSITE KEY is a combination of two or more columns that uniquely identify rows in a table. The combination of columns guarantees uniqueness, though individually uniqueness is not guaranteed. Hence, they are combined to uniquely identify records in a table.

The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key.

What is a Surrogate key?

SURROGATE KEYS is An artificial key which aims to uniquely identify each record is called a surrogate key. This kind of partial key in dbms is unique because it is created when you don’t have any natural primary key. They do not lend any meaning to the data in the table. Surrogate key is usually an integer. A surrogate key is a value generated right before the record is inserted into a table.

FnameLastnameStart TimeEnd Time
AnneSmith09:0018:00
JackFrancis08:0017:00
AnnaMcLean11:0020:00
ShownWillam14:0023:00

Above, given example, shown shift timings of the different employee. In this example, a surrogate key is needed to uniquely identify each employee.

Surrogate keys in sql are allowed when

  • No property has the parameter of the primary key.
  • In the table when the primary key is too big or complicated.

Difference Between Primary key & Foreign key

Primary KeyForeign Key
Helps you to uniquely identify a record in the table.It is a field in the table that is the primary key of another table.
Primary Key never accept null values.A foreign key may accept multiple null values.
Primary key is a clustered index and data in the DBMS table are physically organized in the sequence of the clustered index.A foreign key cannot automatically create an index, clustered or non-clustered. However, you can manually create an index on the foreign key.
You can have the single Primary key in a table.You can have multiple foreign keys in a table.

Summary

  • A key in SQL is an attribute or set of attributes which helps you to identify a row(tuple) in a relation(table)
  • DBMS keys allow you to establish a relationship between and identify the relation between tables
  • Seven Types of DBMS keys are Super, Primary, Candidate, Alternate, Foreign, Compound, Composite, and Surrogate Key.
  • A super key is a group of single or multiple keys which identifies rows in a table.
  • A column or group of columns in a table which helps us to uniquely identifies every row in that table is called a primary key
  • All the keys which are not primary key are called an alternate key
  • A super key with no repeated attribute is called candidate key
  • A compound key is a key which has many fields which allow you to uniquely recognize a specific record
  • A key which has multiple attributes to uniquely identify rows in a table is called a composite key
  • An artificial key which aims to uniquely identify each record is called a surrogate key
  • Primary Key never accept null values while a foreign key may accept multiple null values.

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