How to add a primary key constraint to a column of a table in a database using JDBC API?

You can add a primary key constraint to a column of a table using the ALTER TABLE command.

Syntax

ALTER TABLE table_name
ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

Assume we have a table named Dispatches in the database with 7 columns namely id, CustomerName, DispatchDate, DeliveryTime, Price and, Location with description as shown below:

+--------------+--------------+------+-----+---------+-------+
| Field        | Type         | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName  | varchar(255) | YES  |     | NULL    |       |
| CustomerName | varchar(255) | YES  |     | NULL    |       |
| DispatchDate | date         | YES  |     | NULL    |       |
| DeliveryTime | time         | YES  |     | NULL    |       |
| Price        | int(11)      | YES  |     | NULL    |       |
| Location     | text         | YES  |     | NULL    |       |
| ID           | int(11)      | NO   |     | NULL    |       |
+--------------+--------------+------+-----+---------+-------+

Following JDBC program establishes connection with MySQL database, and adds a primary-key constraint to the column named id.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class Adding_PrimaryKey_Constraint {
   public static void main(String args[]) throws SQLException {
      //Registering the Driver
      DriverManager.registerDriver(new com.mysql.jdbc.Driver());
      //Getting the connection
      String mysqlUrl = "jdbc:mysql://localhost/mydatabase";
      Connection con = DriverManager.getConnection(mysqlUrl, "root", "password");
      System.out.println("Connection established......");
      //Creating the Statement
      Statement stmt = con.createStatement();
      //Query to alter the table
      String query = "ALTER TABLE Sales ADD CONSTRAINT MyPrimaryKey PRIMARY KEY(ID)";
      //Executing the query
      stmt.executeUpdate(query);
      System.out.println("Constraint added......");
   }
}

Output

Connection established......
Constraint added......

Since we added primary constraint on the column named id column, if you get the description of the Sales table using the describe command you can observe that the Key value PRI added opposite to Id.

mysql> describe sales;
+--------------+--------------+------+-----+---------+-------+
| Field        | Type         | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName  | varchar(255) | YES  |     | NULL    |       |
| CustomerName | varchar(255) | YES  |     | NULL    |       |
| DispatchDate | date         | YES  |     | NULL    |       |
| DeliveryTime | time         | YES  |     | NULL    |       |
| Price        | int(11)      | YES  |     | NULL    |       |
| Location     | text         | YES  |     | NULL    |       |
| ID           | int(11)      | NO   | PRI | NULL    |       |
+--------------+--------------+------+-----+---------+-------+
7 rows in set (0.00 sec)