unique key in sql

1ALTER TABLE Persons
2ADD UNIQUE (ID);
3

1create table user(
2    user_id  INT  auto_increment  PRIMARY KEY,
3    username varchar(50) not null,
4    avator  varchar(50),
5    gender  boolean not null,
6    phone varchar(20),
7    unique(username)
8);
9
10create table journey(
11    id INT auto_increment PRIMARY KEY,
12    start varchar(100) not null,
13    dest varchar(100) not null,
14    time date not null,
15    person INT,
16    user_id INT not null,
17    foreign key(user_id) references user(user_id)
18);
19

1USE AdventureWorks2012;  
2GO  
3CREATE TABLE Production.TransactionHistoryArchive4  
4 (  
5   TransactionID int NOT NULL,   
6   CONSTRAINT AK_TransactionID UNIQUE(TransactionID)   
7);   
8GO  
9
10

1DISTINCT
2- select distinct * from employees; ==> retrieves any row if it has at
3least a single unique column.
4- select distinct first_name from employees; ==> retrieves unique names
5from table. (removes duplicates)
6- select distinct count(*) from employees; retrieve number of unique rows
7if any row has at least a single unique data.

1Unique Key:
2Only unique value and also can contain NULL

1CREATE TABLE Students ( 	 /* Create table with a single field as unique */
2    ID INT NOT NULL UNIQUE
3    Name VARCHAR(255)
4);
5
6CREATE TABLE Students ( 	 /* Create table with multiple fields as unique */
7    ID INT NOT NULL
8    LastName VARCHAR(255)
9    FirstName VARCHAR(255) NOT NULL
10    CONSTRAINT PK_Student
11    UNIQUE (ID, FirstName)
12);
13
14ALTER TABLE Students 	 /* Set a column as unique */
15ADD UNIQUE (ID);
16
17ALTER TABLE Students 	 /* Set multiple columns as unique */
18ADD CONSTRAINT PK_Student 	 /* Naming a unique constraint */
19UNIQUE (ID, FirstName);
20