SQL Server - count the number of living grandparents - mysql

Question

I'm homelearning databases and currently trying to solve the following problem:
Select id and name of people who are grandchildren. Also select for
each person the number of their still living grandparents.
My table is looking the following way:
id, name, date_of_birth, date_of_death (is NULL when the person is
alive), gender, father_id, mother_id
I was able to solve the part with selecting the id and name of people who are grandchildren the following way:
SELECT b.name, b.id
FROM persons a
JOIN persons b ON c.father_id = a.id or a.mother_id = a.id
JOIN persons c on p.father_id = c.id
WHERE b.father_id = a.id or b.mother_id = a.id;
However, I am unable to solve the part with the number of still living grandparents for each person.
Could you please help me?
Thanks

Answer 1

This is a question of joining the tables. I think you are all confused by the table aliases on the self-joins. So give them meaningful aliases:
p for the person
pp for the person's parents
gp for the person's grandparents
Then, once you have the JOINs correct, the rest is mostly aggregation:
SELECT p.name, p.id,
SUM(CASE WHEN gp.date_of_death IS NULL THEN 1 ELSE 0 END)
FROM persons p JOIN -- persons
persons pp -- parents
ON pp.id IN (p.father_id, p.mother_id) JOIN
persons gp
ON gp.id IN (pp.father_id, pp.mother_id)
GROUP BY p.name, p.id;
Note the conditional sum, which includes the date of death.

Answer 2

You need to join 3 copies of the table, group by person and count conditionally the number of living grandparents:
SELECT p.id, p.name,
count(case when p2.date_of_death is null then 1 end) living_grandparents
FROM persons p
JOIN persons p1 ON p1.id IN (p.father_id, p.mother_id)
JOIN persons p2 on p2.id IN (p1.father_id, p1.mother_id)
GROUP BY p.id, p.name
For MySql the condition could be simplified to:
sum(p2.date_of_death is null) living_grandparents

Answer 3

Check out the SQL Fiddle for the solution :)
http://sqlfiddle.com/#!9/441ad0/16
select
(
select
count(*)
from
people
where
(
id = p.fatherId
or id = p.motherId
)
and (
fatherId in (
select
id
from
people
where
dod is null
)
or motherId in (
select
id
from
people
where
dod is null
)
)
) as mycount,
p.name,
p.id,
p.fatherId,
p.motherId
FROM
people p
WHERE
p.fatherId in (
SELECT
id
from
people
where
id = p.fatherId
and fatherId in (
SELECT
id
from
people
where
1 = 1
)
)
OR p.motherId in (
SELECT
id
from
people
where
id = p.motherId
and motherId in (
SELECT
id
from
people
where
1 = 1
)
)