mysql - Mysql 2 columns data in a group

Question

I have a table:

id a b
1 1 2
2 2 1
3 2 1
4 3 2
5 2 3

col a b [1 2] is equal to col a b [2 1], so i'd like these data into a group, how can i get a result something looks like:

id a b
5 2 3
3 2 1

Thanks.

Answer 1

你可以做：

select max(id), least(a, b) as a, greatest(a, b) as b
from t
group by least(a, b), greatest(a, b);

(1, 2)如果可以的话，这实际上会(2, 3)为您的数据生成值。

如果要按原始顺序获取数据，则可以将上述内容用作子查询并加入 id：

select t.*
from t join
     (select max(id) as maxid, least(a, b) as a, greatest(a, b) as b
      from t
      group by least(a, b), greatest(a, b)
     ) tmax
     t.id = tmax.maxid;

Answer 2

SELECT *
FROM   TableName
WHERE  (LEAST(a, b), GREATEST(a, b), id) in 
(
    SELECT  LEAST(a, b) as x, 
            GREATEST(a, b) as y, 
            MAX(ID) as id
    FROM    TableName 
    GROUP   BY x, y
)

• SQLFiddle 演示