About keys (ADP)

There are two kinds of keys. A primary key is a set of columns from a table that are guaranteed to have unique values for each row of that table. A primary key is also called a primary key constraint, because it effectively constrains the values you can add to the table: it prevents you from adding a row to the table whose primary key columns are all equal to the corresponding values of some other row in that table.

A foreign key is a correspondence between a set of columns in one table and the set of primary key columns in some other table. When discussing foreign keys, the two participating tables are sometimes called the foreign-key table and the primary-key table. A foreign key is also called a foreign key constraint because it constrains table rows: it ensures that any row you add to the foreign-key table has a corresponding row in the primary-key table. That is, it requires that any row added to the foreign-key table have values in the foreign-key column that correspond to the respective values of the primary key columns for some row in the primary-key table.