PRIMARY KEY uniquely identifies each row in a table.
The column defined as PRIMARY KEY must be UNIQUE (unique value) and NOT NULL (can not contain NULL).
Each table can have only one PRIMARY KEY.
Example PRIMARY KEY in MySQL
Example PRIMARY KEY in ORACLE, SQLSERVER, ACCESS
PRIMARY KEY can consist of multiple columns, for example by the columns 'id' and 'name', then is defined as
PRIMARY KEY can also be defined after creating the table, for that use the ALTER TABLE command
Example with ALTER TABLE PRIMARY KEY
Example ALTER TABLE PRIMARY KEY multiple