mysql> delimiter //
mysql> drop procedure if exists insert_pro//
mysql> create procedure insert_pro(uname varchar(100))
    -> begin
    -> declare idx int;
    -> select id into idx from foo where memo = uname;
    -> if ifnull(idx,0) = 0
    -> then
    -> #insert into foo(memo) values(uname);/*不存在插入*/
    -> #select LAST_INSERT_ID() as not_find; /*插入后返回ID*/
    -> select max(id)+1 as PK from foo; /*不存在返回主键*/
    -> end if;
    -> if idx >0
    -> then
    -> select * from foo where memo=uname;/*存在显示整行*/
    -> end if;
    -> end //
mysql> delimiter ;
mysql> select * from foo;
id      memo
1       f
5       e
9       a
10      b
11      c
mysql> call insert_pro('QQ');
PK
12

mysql> call insert_pro('f');
id      memo
1       f

------解决方案--------------------
create procedure sp_test(a int)
begin
if exists (select 1 from tb where num=a)  
then 
  select id from tb where num=2;
else
  insert into tb(num) values(a);
  select last_insert_id();
end if;
end