GROUP BY 语句可以实现某一列的去重查询。

直接上语句：
select io_dev_id from io_info where (TID=1 AND host_name='yang1') GROUP BY 1；
按照io_dev_id去重查询。
 
p:顺手加上与ORDER BY 和 distinct的区分使用
GROUP BY 是根据列捡选
ORDER BY 是根据列排序
distinct类似于 GROUP BY ，但是只能放在 select 后面，被筛选的字段前面。
如：select distinct a,b,c from tb1;
选出的是a、b、c三列值都相同的数据。
 
摘取的mysql 5.6 reference manual中的内容：
In most cases, a DISTINCT clause can be considered as a special case of GROUP BY. For example, the following two queries are equivalent:
 
SELECT DISTINCT c1, c2, c3 FROM t1
WHERE c1 > const;
SELECT c1, c2, c3 FROM t1
WHERE c1 > const GROUP BY c1, c2, c3;