Keys are used to find the Row with identity and sort the data .They are so many keys present in SQLSERVER
PRIMARY KEY
Keys which are used to find the row with unique identity is known as Primary Key, Primary key is also a Unique Key but doesn't allow the NULL Value, In the below table EmployeeId is consider as primary key
EmployeeId
|
EmployeeName
|
E1
|
Rajesh
|
E2
|
Rajesh
|
Create Table Employee
(
EmployeeId varchar(3) primary key,
EmployeeName varchar(40)
)
UNIQUE KEY
Key which is used to find the uniquely identify the rows of a table is known as Unique Key,Unique Key can allow one NULL value. It doesn't allow duplicate values.
FOREIGN KEY
Key which is used as Reference key for another table and also behaves primary key of the table is known as Foreign Key.In the Below table EmpId is a Primary key for the first table and also it is reference as foreign key in the second table.
EmpId
|
Name
|
Department
|
1
|
Rajesh
|
Computer
|
2
|
Suresh
|
Electronics
|
3
|
Praba
|
Civil
|
Id
|
EmpId
|
PhoneNumber
|
1
|
1
|
3452342
|
2
|
1
|
1234563
|
3
|
1
|
2345612
|
4
|
2
|
2398521
|
5
|
2
|
2323177
|
6
|
2
|
5302994
|
7
|
3
|
3958218
|
COMPOSITE KEY
Composite Key is a combination of more than one columns of a table. It can be a Candidate key, Primary key
CANDIDATE KEY
Any number of columns that are uniquely identify the row in a table is known as Candidate key, Any Candidate key be a Primary Key,But we can select one of them as Primary key.
EmpMailId
|
EmployeeId
|
EmployeeName
|
E1@gma.com
|
E1
|
Rajesh
|
E2@gma.com
|
E2
|
Suresh
|
In the above table,EmpMailId and EmployeeId are Candidate Keys But EmployeeId is consider as Primary Key.
ALTERNATE KEY
Key which can choose as Primary key,But it is a Candidate Key other than primary key for Ex Above Diagram have the EmpMailId is a Alternate Key which can be choose as Primary,but it is a Candidate key not a Primary Key
In this article i explained the concepts of Keys, I hope this will give you some understanding about the concepts of various keys in Sql Server.
No comments:
Post a Comment