我分别执行了下列三个语句，第一个和第二个结果一样．可为什么第三个却和第二个不一样呢，按道理说应该得出一样的结果，为什么第三个还出现重复的答案呢？


select t_teacherno,t_courseno from teaching
结果：
t_teacherno t_courseno
2001050820 2005010001
2001050820 2005010002
2002040820 2005010003
2002040821 2005010004





select t1.t_teacherno,t2.t_courseno
from teaching as t1,teaching as t2
where t1.t_courseno=t2.t_courseno
order by t1.t_courseno

结果：
t_teacherno t_courseno
2001050820 2005010001
2001050820 2005010002
2002040820 2005010003
2002040821 2005010004




select t1.t_teacherno,t2.t_courseno
from teaching as t1,teaching as t2
where t1.t_teacherno=t2.t_teacherno
order by t1.t_teacherno

结果：
t_teacherno t_courseno
2001050820 2005010001
2001050820 2005010002
2001050820 2005010001
2001050820 2005010002
2002040820 2005010003
2002040821 2005010004

第三个和第二个的条件都不一样,原来表里就有那两条记录,根据teacherno连接下当然会出现四条

[此贴子已经被作者于2007-7-8 10:47:19编辑过]

是不一样，可为什么显示重复的结果呀，我还是搞不明白，这代表什么意思？