如何列出MySQL中SET运算符初始化的所有变量？
要列出所有由SET运算符初始化的变量，语法如下：
select * from performance_schema.user_variables_by_thread;
这是设置变量的查询-
mysql> set @FirstName='John'; mysql> set @LastName='Doe';
这是显示由SET运算符初始化的所有变量的列表的查询。该列表包括上面设置的变量-
mysql> select * from performance_schema.user_variables_by_thread;
这将产生以下输出-
+-----------+---------------+----------------+ | THREAD_ID | VARIABLE_NAME | VARIABLE_VALUE | +-----------+---------------+----------------+ | 120 | TotalAmount | 5000 | | 120 | FirstName | John | | 120 | lName | Smith | | 120 | fName | Adam | | 120 | LastName | Doe | +-----------+---------------+----------------+ 5 rows in set (0.00 sec)