Question

Naive question with the answer “No” , I believe, but still would like to ask.

Table_parent

pk_parent_surrogate  
parent_natural_unique_key


Table_child

pk_child_surrogate
child_natural_NOT_unique

Is that true that the only possible declarative relationship among main database vendors is

pk_parent_surrogate ----------<  pk_child_surrogate

and we can’t have declarative constraint or foreign key in other words for pair

parent_natural_unique_key -------< child_natural_NOT_unique

Answer 1

My answer here is based on my MS SQL knowledge – although I believe the same answer is correct for ANSI standards as well, i’m not 100% sure…

YES – you CAN do this as long as you’ve got a unique constraint on the column in your parent table that you want to use as the anchor column for the key.

You can create a FOREIGN KEY constraint as part
of the table definition when you create a table.
If a table already exists, you can add a
FOREIGN KEY constraint, provided that the
FOREIGN KEY constraint is linked to an existing
PRIMARY KEY constraints or UNIQUE constraint in
another, or the same, table. A table can contain
multiple FOREIGN KEY constraints.

And as an example of this sort of key…

use tempdb

CREATE TABLE parent(
    pk int identity primary key, 
    candidate_key int unique not null)

CREATE TABLE child(
    pk int identity primary key, 
    join_key int references parent(candidate_key))

See here for more information.