Can RI Constraint be applied on a Non-Primary Key column?

Question

We have a situation where we need to have a RI constraint on a non-Primary key column of the table.

Scenario:

**TABLE_2**     
**Unique_Key    Expression_Key  Data**
 132             ASDF123    qweqwr
 458             PQRS789    abcd

**TABLE_1**     
**Unique_Key    Expression_Key  Expression_Rule**
  XYZ123             ASDF123    Fname = Simon
  ABC123             ASDF123    Lname = Kelso
  QWE123             ASDF123    Age = 89
  FRU123             ASDF123    Gender = M
  RTY3435            PQRS789    Fname = Michael
  TYR134             PQRS789    Lname = John

The requirement is, if I get the value of 132 - then I need to evaluate the rule of expression_id ASDF123. This Expression ID is not a primary key of Table_1 but its unique for all the rows which are related to that expression.

Can we implement an RI constraint in the above scenario, where the Table_1.Expression_Key = Table_2.Expression_Key if this condition is not met while inserting the data, it should throw an error.

Answer 1

Foreign key constraint can be referencing a unique constraint (it doesn't need to be primary key).