drop procedure if exists pr_param_in;
create procedure pr_param_in
(
  in id int -- in 类型的 MySQL 存储过程参数
)
begin
  if (id is not null) then
   set id = id + 1;
  end if;
  select id as id_inner;
end;

set @id = 10;
call pr_param_in(@id);
select @id as id_out;

mysql> call pr_param_in(@id);
+----------+
| id_inner |
+----------+
|    11 |
+----------+

mysql> select @id as id_out;
+--------+
| id_out |
+--------+
| 10   |
+--------+

drop procedure if exists pr_param_out;
create procedure pr_param_out
(
  out id int
)
begin
  select id as id_inner_1; -- id 初始值为 null
  if (id is not null) then
   set id = id + 1;
   select id as id_inner_2;
  else
   select 1 into id;
  end if;
  select id as id_inner_3;
end;

set @id = 10;
call pr_param_out(@id);
select @id as id_out;

mysql> set @id = 10;
mysql>
mysql> call pr_param_out(@id);
+------------+
| id_inner_1 |
+------------+
|    NULL |
+------------+

+------------+
| id_inner_3 |
+------------+
|     1 |
+------------+

mysql> select @id as id_out;
+--------+
| id_out |
+--------+
| 1   |
+--------+

drop procedure if exists pr_param_inout;
create procedure pr_param_inout
(
  inout id int
)
begin
  select id as id_inner_1; -- id 值为调用者传进来的值
  if (id is not null) then
   set id = id + 1;
   select id as id_inner_2;
  else
   select 1 into id;
  end if;
  select id as id_inner_3;
end;

set @id = 10;
call pr_param_inout(@id);
select @id as id_out;

mysql> set @id = 10;

mysql>
mysql> call pr_param_inout(@id);
+------------+
| id_inner_1 |
+------------+
|     10 |
+------------+

+------------+
| id_inner_2 |
+------------+
|     11 |
+------------+

+------------+
| id_inner_3 |
+------------+
|     11 |
+------------+
mysql>
mysql> select @id as id_out;
+--------+
| id_out |
+--------+
| 11   |
+--------+