SELECT MAX(...) 不正确 returns 存储过程中的 NULL

Question

我有一个 table、ga_sum_1，其中有一列 created_timestamp。当我从 mysql 命令行执行以下查询时：

mysql> select max(created_timestamp) from ga_sum_1;
+------------------------+
| max(created_timestamp) |
+------------------------+
| 2017-11-05 00:59:55    |
+------------------------+
1 row in set (0.00 sec)

如果我从一个非常简单的存储过程中执行相同的操作：

delimiter //
create procedure test()
begin
  select max(created_timestamp) from ga_sum_1;
end//
delimiter ;

mysql> call test();
+------------------------+
| max(created_timestamp) |
+------------------------+
| 2017-11-05 00:59:55    |
+------------------------+
1 row in set (0.00 sec)

但是，当我在存储过程中执行相同操作时：

drop procedure if exists test;

delimiter //

create procedure test()
begin
  declare cursor_end condition for sqlstate '02000';
  declare finished int default 0;
  declare game_id int(11);
  declare user_id int(11);
  declare game_instance_id bigint(20);
  declare currency varchar(15);
  declare created_timestamp timestamp;
  declare wager decimal(18,2);
  declare win decimal(18,2);
  -- cursor
  declare game_action_csr cursor for select * from game_action_view;
  declare continue handler for cursor_end set finished=1;

  -- create view dynamically
select max(created_timestamp) from ga_sum_1;

end//

delimiter ;

mysql> call test();
+------------------------+
| max(created_timestamp) |
+------------------------+
| NULL                   |
+------------------------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

我在最后一个例子中做错了什么？

====编辑====

一些进一步的研究表明它是 declare created_timestamp - 显然混淆了同名列上的 select！

Answer 1

这可能是因为这一行：

  declare created_timestamp timestamp;

Answer 2

作为其他答案的替代，您可以更具体一些：

select max(ga_sum_1.created_timestamp) from ga_sum_1

SELECT MAX(...) 不正确 returns 存储过程中的 NULL

SELECT MAX(...) incorrectly returns NULL in stored procedure

mysql

sql

sql-null