我有一张表格如下:
a | b
---+---
1 | a
2 | a
3 | a
1 | b
3 | b
2 | c
3 | c
它表示多对多关系a a -b。 我希望将所有现有关系设为< - > count(b)< - > a,如:
a1 | a2 | count
----+----+-------
1 | 2 | 1 #1<->a<->2
1 | 3 | 2 #1<->(a,b)<->3
2 | 1 | 1 #duplicate for 1<->a<->2
2 | 3 | 2 #2<->(a,c)<->3
3 | 1 | 2 #duplicate for 1<->(a,b)<->3
3 | 1 | 2 #duplicate for 2<->(a,c)<->3
我已经为单一的a管理了它,但无法弄清楚如何循环使用所有:
SELECT
'1' AS a1,
t1.a AS a2,COUNT(t1.b)
FROM
a_b t1
INNER JOIN(
SELECT
b
FROM a_b
WHERE
a = '1'
) t2
ON
t1.b = t2.b
WHERE t1.a != '1'
GROUP BY t1.a
ORDER BY t1.a;
a1 | a2 | count
----+----+-------
1 | 2 | 1
1 | 3 | 2
如果没有自己交叉加入a_b或通过外部脚本循环,它是否可以实现?
这是SQLFiddle http://www.sqlfiddle.com/#!1/8b53a/1/0
TIA
答案 0 :(得分:2)
我认为这是一个带聚合的基本“连接”查询:
select ab1.a, ab2.a, count(*)
from a_b ab1 join
a_b ab2
on ab1.b = ab2.b and ab1.a <> ab2.a
group by ab1.a, ab2.a
答案 1 :(得分:0)
Gordon Linoff解决方案的一点点补充: 为了不使关系加倍(即1-3和3-1),我添加了where子句:
select ab1.a as a1, ab2.a as a2, count(*)
from a_b ab1 join
a_b ab2
on ab1.b = ab2.b and ab1.a <> ab2.a
where ab1.a < ab2.a
group by ab1.a, ab2.a