You can add one column or column combination as the primary key for a table. A primary key column cannot contain null or duplicate values. You cannot add more than one primary key constraint.
Add a column or column combination as a foreign key in a table. When you define a constraint, you define a foreign key in a table and relate it to a column in a parent table.
Add a column or set of columns as a constraint to define a unique key in a table. A unique key column can contain null values. You can create more than one unique key in table.