In this section you will learn, how to define a unique key in a column of a database table.

Make Unique Column in Database Table

     

In this section you will learn, how to define a unique key in a column of a database table. While declaring unique key in a column we should check whether this column is eligible to store a unique values or not because if the column has been declared as unique then it can't store a duplicate values.  If we ever try to insert the duplicate data in the unique column then it will show an error message to you that you are trying to insert the duplicate value. 

Description of program:

After making a connection with the database firstly compile this program, only after that it will run,  Now give the table name and column name to which you want to make a unique column.  After assigning the unique key to a column the SQL statement is executed with the help of some java methods. If column name becomes unique then shows "Query OK, n rows affected." otherwise, displays a message "Table or column is not found!".

Description of code:

ALTER TABLE  table_name   ADD   UNIQUE(col_name):;
Above code is used to make unique of any column . It takes table name and column name.

Here is the code of program:

import java.io.*;
import java.sql.*;

public class MakeUniqueColumn{
  public static void main(String[] args) {
  System.out.println("Make unique column example!");
  Connection con = null;
  try{
  Class.forName("com.mysql.jdbc.Driver");
  con = DriverManager.getConnection
("jdbc:mysql://localhost:3306/jdbctutorial","root","root");
  try{
  Statement st = con.createStatement();
  BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
  System.out.println("Enter table name: ");
  String table = bf.readLine();
  System.out.println("Enter column name that you want to unique values: ");
  String col = bf.readLine();
  int n = st.executeUpdate("ALTER TABLE "+table+" ADD UNIQUE"+"("+col+")");
  System.out.println("Query OK, "+n+" rows affected.");
  }
  catch (SQLException s){
  System.out.println("Table or column is not found!");
  }
  }
  catch (Exception e){
  e.printStackTrace();
  }
  }
} 

Database Table: Student Table

Stu_id	Stu_name	Stu_sub	Stu_marks
100	vinod	computer	NULL
101	raju	math	NULL
102	ranjan	bio	NULL
103	Aman	phy	NULL

Output of program:

C:\vinod\jdbc\jdbc\jdbc-mysql>javac MakeUniqueColumn.java C:\vinod\jdbc\jdbc\jdbc-mysql>java MakeUniqueColumn Make unique column example! Enter table name: Student Enter column name that you want to unique values: Stu_id Query OK, 4 rows affected.

Make unique values: Student Table

Stu_id	Stu_name	Stu_sub	Stu_marks
100	vinod	computer	NULL
101	raju	math	NULL
102	ranjan	bio	NULL
103	Aman	phy	NULL

Download this example.

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