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) )