Add Unique Key Constraint to a Column in a Database using JDBC API

You can add a unique constraint to a column using the ALTER TABLE command

Syntax

ALTER TABLE table_name
ADD CONSTRAINT MyUniqueConstraint UNIQUE(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) | 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    |       |
+--------------+--------------+------+-----+---------+-------+

Following JDBC program establishes connection with MySQL database, and adds a UNIQUE constraint to the column named CustomerName.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
public class UniqueKey_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 MyUniqueConstraint UNIQUE(ProductName)";
      //Executing the query
      stmt.executeUpdate(query);
      System.out.println("Constraint added......");
   }
}

Output

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

Since we added UNIQUE constraint on the column named ProductName, if you get the description of the Sales table using the describe command you can observe that the Key value UNI added opposite to ProductName.

mysql> describe sales;
+--------------+--------------+------+-----+---------+-------+
| Field        | Type         | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+-------+
| ProductName  | varchar(255) | YES  | UNI | 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)