How to add a NOT NULL constraint to a column of a table in a database using JDBC API?

You can add a not null constraint to a column of a table using the ALTER TABLE command.

Syntax

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

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   | PRI | NULL    |       |
+--------------+--------------+------+-----+---------+-------+

Following JDBC program establishes connection with MySQL database, and adds a NOT NULL constraint to the column named CustomerName.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class NotNull_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 MODIFY CustomerName varchar(255) NOT NULL";
      //Executing the query
      stmt.executeUpdate(query);
      System.out.println("Constraint added......");
   }
}

Output

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

Since we added NOT NULL constraint on the column named id CustomerName, if you get the description of the Sales table using the describe command you can observe the value under the NULL column of CustomerName as NO.

mysql> describe sales;
+--------------+--------------+------+-----+---------+-------+
| Field        | Type         | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName  | varchar(255) | YES  |     | NULL    |       |
| CustomerName | varchar(255) | NO   |     | 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)