Question

我有一个表，代表公共交通服务站点之间的可能连接。看起来像这样：

trip_id | station_id | sequence
--------+------------+---------
  1     |   A        |   1
  1     |   B        |   2
  1     |   C        |   3
  2     |   C        |   1
  2     |   B        |   2
  2     |   A        |   3

现在，我想选择两个站点之间所有可能的连接而没有重复，i。 e。我只需要连接A-> B而不是B-> A。

我想出了这个Join，但是不幸的是有重复项：

SELECT DISTINCT c1.station_id, c2.sation_id 
FROM connections c1, connections c2 
WHERE c1.trip_id = c2.trip_id AND c1.sequence = c2.sequence-1

Answer 1

最简单的方法是对代码进行修改：

SELECT DISTINCT LEAST(c1.station_id, c2.station_id) as station_id1,
       GREATEST(c1.station_id, c2.station_id) as station_id2
FROM connections c1 JOIN
     connections c2 
     c1.trip_id = c2.trip_id AND c1.sequence = c2.sequence - 1

自我加入，无重复

1 个答案: