给定一个表,在这样的有向图中保持边：
CREATE TABLE edges (
from_here int not null,to_there int not null
)
获取特定节点的不同无向链接数量的最佳方法是什么？没有任何重复的定向边缘,也没有任何节点直接链接到自己,我只想避免计数重复的无向边(如(1,2)和(2,1))两次.
这个工作,但NOT IN闻到对我不好：
SELECT COUNT(*)
FROM edges
WHERE from_here = 1
OR (to_there = 1 AND from_here NOT IN (
SELECT to_there
FROM edges
WHERE from_here = 1
))
解决方法
select count(*) from ( select to_there from edges where from_here = 1 union select from_here from edges where to_there = 1 ) as whatever
