ForeignKey Referencing Same Table - mysql

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 mother_id references Person
Foreign Key father_id references Person
and it was asked
"select all who are mothers"
"select those child who are children of 'John Smith' and 'Jane'
and I was puzzled because I was assuming that foreign key would be linked with some other table as usual. But at that point I failed. do some one know the actual answer and reason?

Answer 1

This kind of data structure is called a "Self Referencing Table"
SELECT DISTINCT mothers.*
FROM person
inner join person mothers on person.mother_id = mothers.id
and
SELECT person.*
FROM person
inner join person fathers on person.father_id = fathers.id
inner join person mothers on person.mother_id = mothers.id
WHERE
fathers.name='john smith'
and
mothers.name='jane'

Answer 2

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..

Answer 3

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

Answer 4

SELECT *
FROM Person
WHERE father_id = (SELECT id FROM Person WHERE name = 'John Smith')
AND mother_id = (SELECT id FROM Person WHERE name = 'Jane')

Answer 5

The answer was already given by podiluska, just explaining how it works since it looks like you're new to MySql.
By giving an Alias to a table(like mother or father to the table person) you do something like a pseudo-table, that the MySql interprets as another table, so the join happens normally, just imagine there are 3 tables now. Person, Father and Mother, and they are all linked together by the join.