How to obtain the connection ID of the current connection using the CONNECTION_ID() function?

CONNECTION_ID() is a MySQL built-in function that returns the connection ID of the current connection. The connection ID is also referred as thread ID or process ID. For example:

SELECT CONNECTION_ID();
  -- +-----------------+
  -- | CONNECTION_ID() |
  -- +-----------------+
  -- |             307 |
  -- +-----------------+

SHOW PROCESSLIST; 
  -- +-----+-----------------+-----------------+------+---------+---------+------------------------+------------------+
  -- | Id  | User            | Host            | db   | Command | Time    | State                  | Info             |
  -- +-----+-----------------+-----------------+------+---------+---------+------------------------+------------------+
  -- |   4 | event_scheduler | localhost       | NULL | Daemon  | 1061875 | Waiting on empty queue | NULL             |
  -- | 307 | root            | localhost:52427 | test | Query   |       0 | starting               | show processlist |
  -- +-----+-----------------+-----------------+------+---------+---------+------------------------+------------------+

Reference information of the CONNECTION_ID() function:

CONNECTION_ID(): con
  Returns the connection ID of the current connection.

Arguments, return value and availability:
  con: Return value. The connection ID.
  Available since MySQL 4.0.

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⇑ MySQL Functions on System Information

⇑⇑ MySQL Function References