if sle_1.text<>"" then
SELECT  输入密码
INTO mima
FROM zhuce WHERE  帐号 <>''
else if  sle_1.text="" then
    messagebox("提示","请输入有效的帐号")
end if

试试

正确的是
string zhanghao,mima
zhanghao=sle_1.text
mima=sle_2.text

SELECT password
INTO :mima
FROM zhuce
WHERE  user <>'';
if sle_1.text=""  or sle_2.text='' then
    messagebox("","帐号或密码不能为空！")
    return
end if
if sle_2.text=mima and sle_1.text=zhanghao then
    open(w_main)
    //close(parent)
else
    messagebox("","帐号或密码错误！")
end if