CREATE TABLE Employees (
    Id int NOT NULL,
    PRIMARY KEY (Id),
    ...
);

This will create the Employees table with 'Id' as its primary key. The primary key can be used to uniquely identify the rows of a table. Only one primary key is allowed per table.

A key can also be composed by one or more fields, so called composite key, with the following syntax:

CREATE TABLE EMPLOYEE (
    e1_id INT,
    e2_id INT,
    PRIMARY KEY (e1_id, e2_id)
) 

Many databases allow to make the primary key value automatically increment when a new key is added. This ensures that every key is different.

MySQL

CREATE TABLE Employees (
    Id int NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (Id)
);

PostgreSQL

CREATE TABLE Employees (
    Id SERIAL PRIMARY KEY
);

SQL Server

CREATE TABLE Employees (
    Id int NOT NULL IDENTITY,
    PRIMARY KEY (Id)
);

SQLite

CREATE TABLE Employees (
    Id INTEGER PRIMARY KEY    
);