表A与表B是一对多关系，取表B中任一记录与A关联查询

通过Group By + min方法取得表B中分组后的多条记录中某个字段（一般使用主键或者不重复的列）值最小的记录，然后表A和表B做外连接

select a.id, a.name
  from a
  left join (select id, name
               from b
              inner join (select id, min(key) as key from b group by id) c on b.key =
                                                                              c.key
                                                                          and b.id = c.id) b on a.id = b.id