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SQL Primary Key Constraints with Example
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Primary Key in SQL - How to Use Primary Key in SQL
A primary key uniquely
identifies all the records in the table
In another word, we can say
that the combination of unique and not null constraints is known as the primary
key.
Example
For instance, we have a database of a college with all the
students enrolled in a class and we want to find out the details of a student
whose name is Ram and whose roll number is 101. There is a
possibility that we have another student with the same name and when we tell
the database to fetch details based on that name, it won’t know whose details
we are asking for. Therefore, we need something that uniquely identifies every
record and does not confuse the database and that is where the PRIMARY KEY
comes into the picture.
In the above example, we will have roll number/student id
as a primary key which will uniquely identify each record.
How to Use Primary Key in SQL while Creating table, Alter table
Primary key
on create table
My SQL
CREATE TABLE Student ( ID int NOT NULL, LastName varchar(150) NOT NULL, FirstName varchar(155), Age int, PRIMARY KEY (ID) );
SQL Server / Oracle / MS Access
CREATE TABLE Student ( ID int NOT NULL PRIMARY KEY, LastName varchar(155) NOT NULL, FirstName varchar(155), Age int );
MySQL / SQL Server / Oracle / MS Access
CREATE TABLE Student ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT PK_Student PRIMARY KEY (ID,LastName) );
Primary key
on ALTER TABLE
MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Student ADD PRIMARY KEY (Id) ;
How to Drop
a PRIMARY KEY Constraint
My SQL
ALTER TABLE StudentDROP PRIMARY KEY;
SQL Server / Oracle / MS Access:
ALTER TABLE Student DROP CONSTRAINT PK_Student;
Important point to Remember about Primary Key
· In a Table, We can have Only One primary Key.
· Primary Key must contain a unique value.
· primary key can not be null.
· Primary key can not accept the duplicate value.
· Primary key is the combination of not null and Unique constraint.
Hope!!! The above Tutorial on " SQL Primary key Constraint" is helpful For you...
Team,
QA acharya
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