What does PRIMARY KEY mean and how can we use it in MySQL tables?

#The primary key uniquely identifies each row in the database. PRIMARY KEY must contain unique values ​​and must not contain NULL values. There can only be one primary key in a MySQL table. We can create PRIMARY KEY columns by defining PRIMARY KEY constraints. In order to define the PRIMARY KEY constraint, we have to use the PRIMARY KEY keyword while creating the table, this can be demonstrated in the following example -

Example

The following query we define by putting the "RollNo" column As the primary key, a table named "student" is created-

mysql> Create Table Student(RollNo INT PRIMARY KEY, Name Varchar(20),
   Address Varchar(20), DOB DATE);
Query OK, 0 rows affected (0.16 sec)

Now by describing the table as follows, we can see that "RollNo" has a PRIMARY KEY constraint-

mysql> Describe Student;

+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| RollNo  | int(11)     | NO   | PRI | NULL    |       |
| Name    | varchar(20) | YES  |     | NULL    |       |
| Address | varchar(20) | YES  |     | NULL    |       |
| DOB     | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

4 rows in set (0.03 sec)

Now, "Student "The "RollNo" column of the table must have a unique value and cannot be null.

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