ForeignKey Referencing Same Table

Question

There was an interview test in which below was the table and structure

 Table Person = id, name, dob, dod, mother_id, father_id
 Primary Key (id)
 Foreign Key mo         


        

Answer 1

I was assuming that foreign key would be linked with some other table as usual.

It still is - it references other records in the same Person table.

-- All Mothers
SELECT mom.name
   FROM Person mom
   WHERE EXISTS (SELECT 1 FROM Person WHERE mother_id = mom.id);

-- Children of John Smith and Jane
SELECT kid.name
   FROM Person kid
   INNER JOIN Person mom on kid.mother_id = mom.id
   INNER JOIN Person dad on kid.father_id = dad.id
   WHERE mom.name = 'Jane' AND
   dad.name = 'John Smith';

Have a look at this SQL fiddle here