Answers for "composite unique key in sql"

primary key and unique key in sql

What is the difference between UNIQUE and PRIMARY KEY constraints? 
There should be only one PRIMARY KEY 
in a table whereas there can be
any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL
values whereas Unique key allows NULL values.

unique key in sql

Unique Key:
Only unique value and also can contain NULL

Unique Key in sql

CREATE TABLE Students ( 	 /* Create table with a single field as unique */
    ID INT NOT NULL UNIQUE
    Name VARCHAR(255)
);

CREATE TABLE Students ( 	 /* Create table with multiple fields as unique */
    ID INT NOT NULL
    LastName VARCHAR(255)
    FirstName VARCHAR(255) NOT NULL
    CONSTRAINT PK_Student
    UNIQUE (ID, FirstName)
);

ALTER TABLE Students 	 /* Set a column as unique */
ADD UNIQUE (ID);

ALTER TABLE Students 	 /* Set multiple columns as unique */
ADD CONSTRAINT PK_Student 	 /* Naming a unique constraint */
UNIQUE (ID, FirstName);