Integrity Constraints
=====================
=> are also represented as "constraints".

Why we need to use constraints?
--------------------------------
=> While defining the database objects by following business rules constraints can be used.
=> The advantage with the constraints is:
	avoiding of duplication of the data
Ex: 
create table Books(
sno int,
book_id number,
name_book varchar2(50),
author varchar(50)
);

insert into Books values(1,102938,'Java Fullstack Guide','James Gosling');
insert into Books values(1, 112131, 'Java Fullstack Preparation Guide','Ashok');

select * from Books;

How to define the constraints?
-------------------------------
=> The constraints can be defined in two ways:
	1) Column-level
	2) Table-level
=> If the constraint can be defined within the column definition are called as "column-level constraints".

Syntax:
-------
create table <Table-name>(
	col1 datatype costraint_type,
	col2 datatype
);
=> If the constraint can be defined after all the definitions of columns of the table, then that constraint is called as "Table-level constraint".
Syntax:
-------
create table <Table-name>(
	col1 datatype,
	col2 datatype,
	Constraint <constraint_name_unique> constraint_type(column_name)
);

Here:
	"constraint" is a class.
		to define the "table-level constraints"

Note:
-----
1) For column level constraints, the constraint-name can be assigned by the oracle database automatically.
2) Whereas for the table level constraints the constraint-name can be assigned by the programmer at the time of the definition.

Types of Constraints:
----------------------
=> The total constraints in Oracle are:
	1) unique constraint
	2) not null constraint
	3) check constraint
	4) default constraint
	5) primary key constraint
	6) foreign key constraint

Unique Constraints:
===================
=> to define the table data as "unique" we can use "unique constraint".
=> Disadvantage: not stop to enter null values.

Column-level Unique Constraint:
-------------------------------
Syntax-1:
--------
create table table-name(
	col1 datatype unique,
	col2 datatype
);

create table clients1(
    sno number(6) unique,
    client_id int,
    first_name varchar(50),
    last_name varchar(50),
    unique(client_id)
);
insert into clients1 values(1,10203040,'Sanjy','Sahu');
insert into clients1 values(2,11213141,'Ashok','Guptha');

Table-level Unique constraint:
------------------------------
Syntax-1:
--------
create table table-name(
	col1 datatype,
	col2 datatype,
	unique(col2)
);

Syntax-2:
---------
create table table-name(
	col1 datatype,
	col2 datatype,
	col3 datatype,
	.....
	constraint <constraint-name> unique(column-name);
);


create table products1(
sno number(6),
product_id int,
product_name varchar(50),
price decimal(10,2),
constraint pid unique(product_id)
);

How to apply the unique constraints for the existing table:
------------------------------------------------------------
=> using "alter" command we can define the constraint for the existing table.
Syntax:
	alter table <table-name> add constraint <const-name> unique(column-name);

create table customers(
sno number(6),
customer_id int,
customer_name varchar(50),
location varchar(30),
purchage_cost decimal(10,2)
);

insert into customers values(1,101122,'Gautahm','Hyderabad',1999);

select * from customers;

alter table customers add constraint custid unique(customer_id);

Assignment:
-----------
1) Write a SQL query to disable the constraint which we have created?
2) Write a SQL query to enable the constraint which we have created and disabled?
3) Write a SQL query to add unique constraint for more than one column at a time within the table?