问题描述
假设所有对在它们的镜像的组合中也存在(4,5)
和(5,4)
。但是以下解决方案也可以在没有镜像重复对象的情况下正常工作。
简单的情况
所有连接都可以 ,不可能出现像我在小提琴中添加的那样的复杂情况,我们可以使用此解决方案,而无需在rCTE中重复:
我从获取a_sno
每个组的最小值开始,并具有最小的关联值b_sno
:
SELECT row_number() OVER (ORDER BY a_sno) AS grp
, a_sno, min(b_sno) AS b_sno
FROM data d
WHERE a_sno < b_sno
AND NOT EXISTS (
SELECT 1 FROM data
WHERE b_sno = d.a_sno
AND a_sno < b_sno
)
GROUP BY a_sno;
结果:
grp a_sno b_sno
1 4 5
2 9 10
3 11 15
我避免使用分支和重复的(重复的)行-长链可能 会 更昂贵。我ORDER BY b_sno LIMIT 1
在相关子查询中使用了递归CTE。
性能的关键是匹配索引,它已经由PK约束提供了:反之则PRIMARY KEY (a_sno,b_sno)
不行 :(b_sno, a_sno)
-
WITH RECURSIVE t AS ( SELECT row_number() OVER (ORDER BY d.a_sno) AS grp , a_sno, min(b_sno) AS b_sno – the smallest one FROM data d WHERE a_sno < b_sno AND NOT EXISTS ( SELECT 1 FROM data WHERE b_sno = d.a_sno AND a_sno < b_sno ) GROUP BY a_sno )
, cte AS ( SELECT grp, b_sno AS sno FROM t
UNION ALL SELECT c.grp , (SELECT b_sno – correlated subquery FROM data WHERE a_sno = c.sno AND a_sno < b_sno ORDER BY b_sno LIMIT 1) FROM cte c WHERE c.sno IS NOT NULL ) SELECT * FROM cte WHERE sno IS NOT NULL – eliminate row with NULL UNION ALL – no duplicates SELECT grp, a_sno FROM t ORDER BY grp, sno;
不太简单的情况
从根(最小sno
)到一个或多个分支,可以按升序到达所有节点。
这次,获得 所有 更大sno
和重复数据删除的节点,这些节点可能在最后访问了多次UNION
:
WITH RECURSIVE t AS (
SELECT rank() OVER (ORDER BY d.a_sno) AS grp
, a_sno, b_sno -- get all rows for smallest a_sno
FROM data d
WHERE a_sno < b_sno
AND NOT EXISTS (
SELECT 1 FROM data
WHERE b_sno = d.a_sno
AND a_sno < b_sno
)
)
, cte AS (
SELECT grp, b_sno AS sno FROM t
UNION ALL
SELECT c.grp, d.b_sno
FROM cte c
JOIN data d ON d.a_sno = c.sno
AND d.a_sno < d.b_sno -- join to all connected rows
)
SELECT grp, sno FROM cte
UNION -- eliminate duplicates
SELECT grp, a_sno FROM t -- add first rows
ORDER BY grp, sno;
与第一种解决方案不同,我们在这里没有得到带有NULL的最后一行(由相关子查询引起)。
解决方法
我在Postgres中有下表,在两列a_sno
和中有重叠的数据b_sno
。
create table data
( a_sno integer not null,b_sno integer not null,PRIMARY KEY (a_sno,b_sno)
);
insert into data (a_sno,b_sno) values
( 4,5 ),( 5,4 ),6 ),( 6,7 ),( 7,( 9,10),13),(10,9 ),(13,14),(14,(11,15),(15,11);
从前6行可以看到,两列中的数据值4,5,6和7相交/重叠,需要将其划分为一组。第7-16行和第17-18行将分别标记为组2和3。
结果输出应如下所示:
group | value
------+------
1 | 4
1 | 5
1 | 6
1 | 7
2 | 9
2 | 10
2 | 13
2 | 14
3 | 11
3 | 15