Implementation of many to many relationship in java.

In this tutorial i will show you how to implement many to many relationship in sql when tables are in normalize form.In normalization of a database when two tables have many to many relationship we broke the tables into three tables two tables are original tables and one is a junction table which connects the two tables which have many to many relation.
In that way we break many to many realtion nto 1 to many relation.I will give you an example that clears the concept.

For example you have two entities or tables which are student and courses.Now one student can take many courses and similarly a course is taken by many students so these entities have many to many relation.

Okay lets discuss what are the problems arise if we implement tables as it is without breaking the many to many relationship,So these are the problems that can arise

 -Update anomaly
-Delete anomaly
-Data repetition
-Insert anomaly

To avoid these problems these tables should be normalized by creating the junction table,now these are the steps to normalize these tables.

1)Create a table with name student_course which has the composite primary key of both the id's of student and course table for example if student and courses tables have the attributes.
Student(std_id,name,address)
Courses(cr_id,name)

so the student_course table has the following attribute

student_course(std_id,cr_id , date)

2)In student_course table std_id and cr_id is composite pk which is taken by student and courses tables as a foreign key.

So in that way you succesfully normalized your tables.


Now the main problem is what are the queries to implement this concept which is the very big problem
just follow these simple steps to implement this concept.

1)create table of student with student id as primary key and auto increment.
2)create table of courses with course id as primary key and auto increment.
3)create the junction table using foreign key of student and courses tables id and make it composite pk.
4)First you have to execute the insert query of student table that inserted a record in student table with id 1.
5)insert some courses in courses tables.
6)Now very importantly Execute this query "SET @last_id=LAST_INSERT_ID()"
this query gives you the last inserted id in the student table,So you will use this variable to insert values int the student_courses table.You should run this insert queries in loop so that many courses are registered for one student.

Here are the queries

String sql="insert into student(`fname`,`lname`,`email`,`gender`,`desc`,`subject`,`city`)"+"values('"+fname+"','"+lname+"','"+email+"','"+gender+"','"+desc+"','"+s+"','"+city+"')";


         conn.createStatement().executeUpdate(sql);

         conn.createStatement().executeUpdate("SET @last_id=LAST_INSERT_ID();");

         for(int i=0;i<subject.length; i++){
         String sql2="insert into std_sbj(`std_id`,`sbj_id`)values(@last_id,'"+Integer.parseInt(subject[i])+"')";

        conn.createStatement().executeUpdate(sql2);

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How to get values from user in java

In this tutorial i will show you how to code a program in java that takes values from user and then print on console,so lets start.
These are the steps that you follow:
1) I assumed you are able to create a java project in eclipse,after creating the project you see your project on left bar of your eclipse interface,now if you click on the project you will see the folder with name "Source Packages" if you click on this folder it will show you a package with name "Default package",now right click on that and create a new java class with any name,In my case i am going to make a class with name HellowWorld.

2) After creating the class you have to write the following code.For example

             public class HelloWorld{

                     String userinput;

                  public   HellowWorld(){
                         
                           
                             

}
                      //Now we make getter setter for this variable

              public void setUseinput(String up){

                                     userinput=up;
}

public String getUserinput(){

               return userinput;

}

public static void main(String arg[]){

       HellowWorld obj=new HellowWorld();

                     Scanner input=new Scanner(System.in);//this is used for get values from keyboard.

       System.out.println("Enter the String");
       int var=input.nextInt();

     obj.setUserinput(var);
           System.out.println("User entered string"+obj.getUserinput);

}

}

So when you run this program it will take input from keyboard and print on the console.

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