When creating a table, adding a column to a table, or updating a column in a table, the column can have the Key flag set to PRI (PRIMARY KEY) and empty.

In this example, the "id" column is set as the PRIMARY KEY and the "name" column has no key set.

~]$ mysql -e "describe freekb.table001"
Field   Type          Null    Key     Default  Extra
id      int(11)       NO      PRI     NULL     auto_increment
name    varchar(200)  YES             NULL

Each table can have only one column set as the PRIMARY KEY.

When a column has the PRIMARY KEY set, the column must adhere to these rules:

• The column must contain unique values (no duplicate identical values)
• The column cannot contain NULL (empty) values

Often, the "id" column has the PRIMARY KEY set and auto_increment, so that when a new row is added to the table, the ID is automatically incremented by 1, which ensure the "id" column contains unique values and does not contain any null/empty values.