The line that is not used to turn on the event scheduler is . . . . . . . .

Solution (By Examveda Team)

The correct answer is D: eventscheduler = ON.
Let's understand why and how to correctly turn on the MySQL event scheduler.

The event scheduler in MySQL is a special feature that allows you to schedule tasks (like running a stored procedure or an SQL statement) to happen automatically at specific times or intervals, similar to `cron` jobs in Linux.

To turn on the event scheduler, you need to set the `event_scheduler` system variable to `ON`.

Here's why the options are correct or incorrect:

Option D: eventscheduler = ON
This is the incorrect way because the system variable name in MySQL is `event_scheduler` (with an underscore), not `eventscheduler` (without an underscore). MySQL is strict about variable names. Therefore, this line will not turn on the event scheduler. This is the line that is not used for the purpose, making it the correct answer to the question.

Option A: event_scheduler = ON
This line is used in the MySQL configuration file (like `my.cnf` on Linux or `my.ini` on Windows) under the `[mysqld]` section. When MySQL starts, it reads this file, and if this line is present, the event scheduler will be turned `ON` automatically.

Option B: SET GLOBAL event_scheduler = ON;
This is a SQL command used to turn on the event scheduler globally for the current running MySQL server instance. The `GLOBAL` keyword means the setting applies to all current and future sessions. This change is active immediately but will be lost if the server restarts unless it's also set in the configuration file (Option A).

Option C: SET @@GLOBAL.event_scheduler = ON;
This is another SQL command that achieves the same result as Option B. `@@GLOBAL.variable_name` is an alternative syntax to refer to a global system variable. It also turns on the event scheduler globally for the current running instance.

In summary, Options A, B, and C are valid ways to enable the event scheduler, while Option D uses an incorrect variable name and thus does not work.