A primary key uniquely identify a row in a table. One or more columns may be identified as the primary key. The values in a single column used as the primary key must be unique (like a person’s social security number). When more than one column is used, the combination of column values must be unique.
When creating the contacts table described in Create a basic MySQL table, the column contact_id can be made a primary key using PRIMARY KEY(contact_id) as with the following SQL command:
CREATE TABLE `test1` (
contact_id INT(10),
name VARCHAR(40),
birthdate DATE,
PRIMARY KEY (contact_id)
);
Additional columns can be identified as part of the primary key with a comma separated list in the PRIMARY KEY command, like PRIMARY KEY (contact_id, name).
