unique key in dbms code example

Example 1: sql make sure every user is unqiuew

create table user(
    user_id  INT  auto_increment  PRIMARY KEY,
    username varchar(50) not null,
    avator  varchar(50),
    gender  boolean not null,
    phone varchar(20),
    unique(username)
);

create table journey(
    id INT auto_increment PRIMARY KEY,
    start varchar(100) not null,
    dest varchar(100) not null,
    time date not null,
    person INT,
    user_id INT not null,
    foreign key(user_id) references user(user_id)
);

Example 2: unique key

CREATE TABLE students  
(  
S_Id int NOT NULL,  
LastName varchar (255) NOT NULL,  
FirstName varchar (255),  
City varchar (255),  
UNIQUE (S_Id)  
)

Example 3: unique key

Unique Key:
Only unique value and also can contain NULL