Question

What is a Primary Key in SQL?

Answer 1

The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint.
A table in SQL is strictly restricted to have one and only one primary key, which is comprised of single or multiple fields (columns).

CREATE TABLE Students ( 	 /* Create table with a single field as primary key */
    ID INT NOT NULL
    Name VARCHAR(255)
    PRIMARY KEY (ID)
);

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

ALTER TABLE Students 	 /* Set a column as primary key */
ADD PRIMARY KEY (ID);

ALTER TABLE Students 	 /* Set multiple columns as primary key */
ADD CONSTRAINT PK_Student 	 /*Naming a Primary Key*/
PRIMARY KEY (ID, FirstName);