PRIMARY KEY definition in MySQL CREATE TABLE statement

Question

What\'s the difference between this code:

CREATE TABLE samples (
  sampleid INT(11) NOT NULL AUTO_INCREMENT,
  sampledate DATE NOT NULL,
  location VARCHAR(25) N         


        

Answer 1

MySQL allows uses the PRIMARY KEY directive to allow you to set the Primary Key dynamically. Supplying PRIMARY KEY as an argument to the constructor can only be called on creating the column. PRIMARY KEY(X), PRIMARY KEY(Y), PRIMARY KEY(Z) allows for changing the primary keys on subsequent queries.