read me zhengjj
st=>start: 开始
e=>end: 登录
io1=>inputoutput: 输入用户名密码
sub1=>subroutine: 数据库查询子类
cond=>condition: 是否有此用户
cond2=>condition: 密码是否正确
op=>operation: 读入用户信息
st->io1->sub1->cond
cond(yes,right)->cond2
cond(no)->io1(right)
cond2(yes,right)->op->e
cond2(no)->io1