A$($n$)$ foreign key is a column $($or collection of columns$)$ in one table whose value is required to match the value of the primary key for another table.

entity integrity integrity is the property that states that no record can exist in the database with a value in the field other than one of its legal values.

The Legal$-$values

rule states that no

column that is part of the primary key can accept null values.

What is an index key? The field or fields on which the index is built

What is a multiple$-$field index? Indexes whose key is a combination of fields