在存储过程中我要实现一个IF的嵌套语句查询，类似与
if（）
{
if（）
{
......
}
else
{
......
}
}
else
{
......
}
请问这样的SQL语句要怎么写啊？
create procedure `test2`
(IN cusername <a href="https://www.baidu.com/s?wd=VARCHAR&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1dWPj6knWw9PvDzPAc4nH0z0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6K1TL0qnfK1TL0z5HD0IgF_5y9YIZ0lQzqlpA-bmyt8mh7GuZR8mvqVQL7dugPYpyq8Q1D3rjnYnWbYnHDzPjmzrHTkPj6" target="_blank" class="baidu-highlight">VARCHAR</a>(50),
IN cpassword <a href="https://www.baidu.com/s?wd=VARCHAR&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1dWPj6knWw9PvDzPAc4nH0z0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6K1TL0qnfK1TL0z5HD0IgF_5y9YIZ0lQzqlpA-bmyt8mh7GuZR8mvqVQL7dugPYpyq8Q1D3rjnYnWbYnHDzPjmzrHTkPj6" target="_blank" class="baidu-highlight">VARCHAR</a>(50),
IN clastloginio <a href="https://www.baidu.com/s?wd=VARCHAR&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1dWPj6knWw9PvDzPAc4nH0z0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6K1TL0qnfK1TL0z5HD0IgF_5y9YIZ0lQzqlpA-bmyt8mh7GuZR8mvqVQL7dugPYpyq8Q1D3rjnYnWbYnHDzPjmzrHTkPj6" target="_blank" class="baidu-highlight">VARCHAR</a>(50)
)BEGIN if exists(select * from mt_userlogin where username=cusername) then
-- if exists(select 1) then if(1=1) THEN
select 1;
<a href="https://www.baidu.com/s?wd=ELSE&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1dWPj6knWw9PvDzPAc4nH0z0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6K1TL0qnfK1TL0z5HD0IgF_5y9YIZ0lQzqlpA-bmyt8mh7GuZR8mvqVQL7dugPYpyq8Q1D3rjnYnWbYnHDzPjmzrHTkPj6" target="_blank" class="baidu-highlight">ELSE</a>
select 2;
end if;
<a href="https://www.baidu.com/s?wd=ELSE&tn=44039180_cpr&fenlei=mv6quAkxTZn0IZRqIHckPjm4nH00T1dWPj6knWw9PvDzPAc4nH0z0ZwV5Hcvrjm3rH6sPfKWUMw85HfYnjn4nH6sgvPsT6K1TL0qnfK1TL0z5HD0IgF_5y9YIZ0lQzqlpA-bmyt8mh7GuZR8mvqVQL7dugPYpyq8Q1D3rjnYnWbYnHDzPjmzrHTkPj6" target="_blank" class="baidu-highlight">ELSE</a>
select -7,'Username not found';
end if;
end;