Primary Key Constraint:
=======================
=> Primary key constraint is the combination of unique constraint and not null constraint.
=> All other constraints like unique, not null and check etc are allowed to define on multiple fields/columns of the same table.
But the primary key constraint can allow to define only once in a table.
=> we can able to use the primary key constraint at column level and also at table level.

create table dummy
(
	sno number(10) unique,
	id int unique,
	name varchar2(50) not null,
	age int(2) check (age >= 18 and age <= 60),
	gender varchar2(15) not null,
	location varchar(30) not null
);

Column level primary key constraint:
------------------------------------
Syntax:
	create table table-name
	(
		col1 datatype primary key,
		col2 datatype,
		....
	);	


create table products102
(
    sno number(10) unique,
    product_id int primary key,
    product_name varchar2(50) not null,
    price decimal(10,2) check(price >= 1)
);

insert into products102 values(1,112233,'Laptop',59999);

insert into products102 values(2,113233,'Smart Phone',29999);
insert into products102 values(3,102030,'Speakers',69999);

select * from products102;

Table level primary key constraint:
-----------------------------------
Syntax:
------
	create table table-name
	(
		col1 datatype,
		col2 datatype,
		constraint <constraint-name> primary key(column-name)
	);

Ex:
create table employee121
(
    sno number(10) unique not null,
    emp_id int,
    emp_name varchar2(50) not null,
    salary decimal(10,2) check(salary > 0),
    location varchar2(30) not null,
    constraint pk_const primary key(emp_id)
);

Defining of the primary key constraint for the existing table:
---------------------------------------------------------------
=> we can use the alter command
Syntax:
	alter table table-name add constraint <const-name> primary key(column-name);

Enabling/Disabling of primary key constraint:
---------------------------------------------
Syntax:
	alter table table-name enable primary key;
	alter table table-name disable primary key;


Q: Write a query to display all constraints?
============================================
Syntax:

	select * from user_constraints;

Q: Write a query to display all constraints of the given/particular table?
==========================================================================
Syntax:
	select * from user_constraints where table_name = 'products102';

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