Practically understanding on SQL constraints in SQL server.

===Not Null
CREATE TABLE employees1 (
id INT NOT NULL ,
name VARCHAR(50) NOT NULL,
age INT);

select * from employees1

insert into employees1(id,name,age) values(102,'lav',40)

insert into employees1(id,name,age) values(102,'lav',40)
====UNIQUE Constraint
CREATE TABLE students (
id int,
email VARCHAR(50),
phone VARCHAR(20));

select * from students

=========PRIMARY KEY Constraint
CREATE TABLE customers (
id INT PRIMARY KEY,
name VARCHAR(50),
address VARCHAR(100)
);

insert into customers values(104,'lavlesh',null)

select * from customers
===FOREIGN KEY Constraint
CREATE TABLE orders (
id INT,
customer_id INT,
order_date DATE,
FOREIGN KEY (id) REFERENCES customers(id)
);

drop table orders

select * from orders

insert into orders values(103,105, '2023-03-10')