How to retrieve all grand parents at once - sql

Question

I have two simple tables in SQLITE3 (family bonds):
"persons" (id, fname, lname)
"relationships" (parent/child)
I would like to get each grand children along with all their grand parents (from 1 to 4 of them depending on the grand child) so that 1 row result =
Distinct Grand child, Grand parent 1, Grand parent 2, Grand parent 3, Grand parent 4
Thanks to Caius Jard, I've been able to get each child and its grand parents in another star overflow question. However, so far I have:
1 line = Grand child, 1 grand parent (so it needs up to 4 lines to get all grand parents of a child).
SELECT c.fname, c.lname, gp.fname, gp.lname
FROM relations p_c
INNER JOIN relationships gp_p ON gp_p.child = p_c.parent
INNER JOIN persons gp ON gp.id = gp_p.parent
INNER JOIN persons c ON c.id = p_c.child
ORDER BY c.id;
How could I edit this so that I get each grand child along with all the grand parents in one single row result?
If possible only using SELECT (+ count/distinct), WHERE (+in/exists), INNER/LEFT JOIN, GROUP BY (+having), ORDER BY.

Answer 1

We can turn rows into columns by a technique known as conditional aggregation:
Suppose you have a result set:
x, y
1, hello
2, goodbye
If you wrote this:
SELECT x,
CASE WHEN x = 1 THEN y END as x1,
CASE WHEN x = 2 THEN y END as x2
FROM ...
You would get this:
x, x1, x2
1, hello, NULL
2, NULL, goodbye
There is only one value in a column, the rest are null. If you then use e.g. MAX, which discards nulls:
SELECT
MAX(CASE WHEN x = 1 THEN y END) as x1,
MAX(CASE WHEN x = 2 THEN y END) as x2
FROM ...
You would get
x1, x2
hello, goodbye
We've had to drop the x, because we can't put it in there otherwise we'd have to group it, which would prevent rows collapsing down
Now to your query, we probably have to add other info in because we need something to CASE WHEN on. You don't indicate that we can arbitrarily throw a row number in there. Hopefully persons contains a gender indicator because 4 grandparents are the result of two males and two females arising from one male and one female (so we have MM - father's father, MF - father's mother, FM and FF etc)
SELECT ...
FROM relations p_c
INNER JOIN relationships gp_p ON gp_p.child = p_c.parent
INNER JOIN persons gp ON gp.id = gp_p.parent
INNER JOIN persons p ON p.id = p_c.parent
INNER JOIN persons c ON c.id = p_c.child
We can now query the gender of each level of family tree:
SELECT c.name as child,
CASE WHEN gp.gender = 'm' AND p.gender = 'm' THEN gp.name END AS fathersfather,
CASE WHEN gp.gender = 'm' AND p.gender = 'f' THEN gp.name END AS fathersmother,
CASE WHEN gp.gender = 'f' AND p.gender = 'm' THEN gp.name END AS mothersfather,
CASE WHEN gp.gender = 'f' AND p.gender = 'f' THEN gp.name END AS mothersmother
FROM relations p_c
INNER JOIN relationships gp_p ON gp_p.child = p_c.parent
INNER JOIN persons gp ON gp.id = gp_p.parent
INNER JOIN persons p ON p.id = p_c.parent
INNER JOIN persons c ON c.id = p_c.child
This will give the child and then a diagonal array of the grandparent names. You can squish it to a row per child with:
SELECT c.name as child,
MAX(CASE WHEN gp.gender = 'm' AND p.gender = 'm' THEN gp.name END) AS fathersfather,
MAX(CASE WHEN gp.gender = 'm' AND p.gender = 'f' THEN gp.name END) AS fathersmother,
MAX(CASE WHEN gp.gender = 'f' AND p.gender = 'm' THEN gp.name END) AS mothersfather,
MAX(CASE WHEN gp.gender = 'f' AND p.gender = 'f' THEN gp.name END) AS mothersmother
FROM relations p_c
INNER JOIN relationships gp_p ON gp_p.child = p_c.parent
INNER JOIN persons gp ON gp.id = gp_p.parent
INNER JOIN persons p ON p.id = p_c.parent
INNER JOIN persons c ON c.id = p_c.child
GROUP BY c.name

Answer 2

Instead of 4 columns for the grand parents, I would suggest to use GROUP_CONCAT() and get 1 column with all the grandparents:
SELECT p.fname, p.lname,
GROUP_CONCAT(gp.fname || ' ' || gp.lname, '|')
FROM persons p
INNER JOIN relationships r1 ON r1.child = p.id
INNER JOIN relationships r2 ON r2.child = r1.parent
INNER JOIN persons gp ON r2.parent = gp.id
GROUP BY p.id
The code is by far simpler.
Use LEFT joins instead of INNER joins if you want all the persons returned, even the ones without grandparents.

Answer 3

Assuming there are four grandparents, you can enumerate them and aggregate:
SELECT fname, lname,
MAX(CASE WHEN seqnum = 1 THEN grandparent_name END) as grandparent_1,
MAX(CASE WHEN seqnum = 2 THEN grandparent_name END) as grandparent_2,
MAX(CASE WHEN seqnum = 3 THEN grandparent_name END) as grandparent_3,
MAX(CASE WHEN seqnum = 4 THEN grandparent_name END) as grandparent_4
FROM (SELECT c.id, c.fname, c.lname,
gp.fname || ' ' || gp.lname as grandparent_name,
ROW_NUMBER() OVER (PARTITION BY c.id ORDER BY gp.fname, gp.lanme) as seqnum
FROM relations p_c JOIN
relationships gp_p
ON gp_p.child = p_c.parent JOIN
persons gp
ON gp.id = gp_p.parent
persons c
ON c.id = p_c.child
) cgp
GROUP BY fname, lname, id;