mysql>use test; mysql> drop procedure if exists pr_param_in; Query OK, 0 rows affected, 1 warning (0.01 sec) mysql> delimiter // mysql> create procedure pr_param_in(in id int) -> begin -> if (id is not null) then -> set id=id+1; -> end if; -> select id as id_inner; -> end; -> // Query OK, 0 rows affected (0.03 sec) mysql> delimiter ; mysql> set @id=10; Query OK, 0 rows affected (0.00 sec) mysql> call pr_param_in(@id); +----------+ | id_inner | +----------+ | 11 | +----------+ 1 row in set (0.00 sec) Query OK, 0 rows affected (0.00 sec) mysql> select @id as id_out; +--------+ | id_out | +--------+ | 10 | +--------+ 1 row in set (0.00 sec)
## 可以看到用户变量@id传入值为10，执行存储过程后，在过程内部值为：11（id_inner）,
## 但外部变量值依旧为：10（id_out）mysql> drop procedure if exists pr_param_out; Query OK, 0 rows affected, 1 warning (0.01 sec) mysql> delimiter // mysql> create procedure pr_param_out(out id int) -> begin -> select id as id_inner_1; -> 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; -> // Query OK, 0 rows affected (0.01 sec) mysql> delimiter ; mysql> set @id=10; Query OK, 0 rows affected (0.00 sec) mysql> call pr_param_out(@id); +------------+ | id_inner_1 | +------------+ | NULL | +------------+ 1 row in set (0.01 sec) +------------+ | id_inner_3 | +------------+ | 1 | +------------+ 1 row in set (0.01 sec) Query OK, 0 rows affected (0.01 sec) mysql> select @id as id_out; +--------+ | id_out | +--------+ | 1 | +--------+ 1 row in set (0.00 sec)
## 可以看出，虽然我们设置了用户定义变量@id为10，传递@id给存储过程后，在存储过程内部，
## id的初始值总是 null（id_inner_1）。最后id值（id_out=1）传回给调用者。mysql> drop procedure if exists pr_param_inout; Query OK, 0 rows affected, 1 warning (0.01 sec) mysql> delimiter // mysql> create procedure pr_param_inout(inout id int) -> begin -> select id as id_inner_1; -> 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; -> // Query OK, 0 rows affected (0.01 sec) mysql> delimiter ; mysql> set @id=10; Query OK, 0 rows affected (0.00 sec) mysql> call pr_param_inout(@id); +------------+ | id_inner_1 | +------------+ | 10 | +------------+ 1 row in set (0.00 sec) +------------+ | id_inner_2 | +------------+ | 11 | +------------+ 1 row in set (0.00 sec) +------------+ | id_inner_3 | +------------+ | 11 | +------------+ 1 row in set (0.01 sec) Query OK, 0 rows affected (0.01 sec) mysql> select @id as id_out; +--------+ | id_out | +--------+ | 11 | +--------+ 1 row in set (0.00 sec)
## 从结果可以看出：我们把 @id(10）传给存储过程后，存储过程最后又把计算结果值11(id_inner_3)
## 传回给调用者。MySQL存储过程inout参数的行为跟C语言函数中的引用传值类似。
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