Referantial integrity deals in parent-child relationship between tables.

The condition is that the child table must have only the column values that are present in the parent table. A new value that is not presant in the parent table cannot be updated into the child table. Likewise an existing value cannot be deletted from the parent table if the child table consists its corresponding row.

Referential Integrity Constraint is used to specify interdependencies between relations. This constraint specifies a column or list of columns as a foreign key of the referencing table.

A foreign key means the values in one table must also appear in another table. The foreign key in the child table will generally reference a primary key in the parent table.The referencing table is called the child table & referenced table is called the parent table.

Syntax: constraint <constraint name> references <primary table><primary key>