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

You can always have foreign key that link to the same table.. There is not problem in that..

See, suppose you are storing a person record in the table, now that person would be having mother and father. And mother and father both are themselves a person.. so, they are itself a record of the same table..

Suppose you have following two records in Person table: -

id     name           age         mother_id
1      xyz            24          5
5      abc            57          6

So, from the above table you see that, person with id = 5 is actually mother of person with id = 1.. So, it is a foreign key referencing the same table..

So, here rather than performing a join operation with a different table, you have to perform join with the same table..

SELECT p2.id FROM
Person p1 join Person p2
WHERE p1.mother_id = p2.id

This query will select the record of mother of your current record..