DEV Community: Anand Singh

Role of Primary key in DBMS

Anand Singh — Wed, 09 Dec 2020 15:03:10 +0000

An entity can contain multiple keys but the key which is most suitable from those lists become a primary key.

In the EMPLOYEE table, id can be primary key since it is unique for each employee.

Main features are-
1-It must contain a unique value.
2-It cannot contain null values.

dbms #primarykey

String important points

Anand Singh — Tue, 29 Sep 2020 05:34:04 +0000

String s1="java";
String s2="coder";

//Sop means System.out.println(for print)

Sop(s1+s2); //output-javacoder

Sop(s1+10); // java10
//10 is treated like a string.

Sop(s1+10+20); //java1020

Sop(10+20+s1); //30java
//First 10 and 20 were added and then 30 concat with s1.

note-Left to right operation takes place with the plus operator ... first addition will happen and then concat.

Sop(10+s1+20); //10java20

Sop(s1+20/10); //java2
//First division took place and then concat method was used

Note--BODMAS rule is used in Java ... if you do not know the bodmas rule then do a Google search.

Sop(s1+10-5); //error

//first s1+10 =java10
then java10-5
java10 -5 is not possible, so we will get error.

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Important points for Java interface

Anand Singh — Mon, 21 Sep 2020 11:49:37 +0000

interface I1
{
public void show();
//interface tells the class what to do
}
class Test implements I1
//for interface use implements keyword
{
public void show()
//It is necessary to create body of abstract method in child class
{
System.out.println("1");
}

public static void main(String[] args)
{
//We cannot create an object of interface so we have created the object of test class here
Test t=new Test();
t.show();

}
}