我有一张桌子people
:
ID name surname mother_id father_id
--------------------------------------------
1 John smith null null
2 kate m null null
3 philip smith 2 1
4 dimitr smith 7 3
等
我尝试了此查询。
SELECT
p.name, P.surname,
c.name as 'Mothers name', c.surname as 'Mothers surname',
gm.name as 'Grandmother name', gm.surname as 'Grandmother surname'
FROM
dbo.people AS p
JOIN
dbo.people AS c ON (c.mother = p.Id)
JOIN
dbo.people AS gm ON (gm.mother = c.Id)
但是它并没有真正返回我的期望
答案 0 :(得分:1)
ON
子句存在的问题:
select p.name, p.surname,
coalesce(pm.name, '') as mothername, coalesce(pm.surname, '') as mothersurname,
coalesce(pf.name, '') as grandmothername,
coalesce(pf.surname, '') as grandmothersurname
from people p left join
people pm
on pm.id = p.mother_id left join -- use mother_id from people p
people pf
on pf.id = p.father_id; -- use father_id from people p;
这里是db-fiddle。