关于sqlite数据库连接查询问题?

Question

表1：如下
CREATE TABLE student (sid integer, sname text, age integer);
sqlite> select * from student;
1|zhang|22
2|li|21
3|wang|19
4|xue|19
5|zhao|20
表2：如下
CREATE TABLE achievements (cid integer, sid integer, course text, results integer);
2|1|shuxue|85
4|2|shuxue|90
6|3|shuxue|82
8|4|shuxue|72
10|5|shuxue|100

我要查询年龄为20的学生的成绩该怎么写连接sql语句？？？
sqlite> select a.results from achievements as a inner join student as s where s.age=20;
85
90
82
72
100
为什么上面的语句查出来的不是100这个数，而是把所有的成绩全打出来了？？正确的语句该怎么写？？？

Answer 1

少一个连接条件select a.results from achievements as a inner join student as s where s.age=20;改为select a.results from achievements as a inner join student as s on a.sid = s.sid where s.age=20;