CONNECTION_ID() returns the connection ID (thread ID) for the connection. : CONNECTION_ID « Information Functions « MySQL Tutorial

1. MySQL Tutorial
2. Information Functions
3. CONNECTION_ID

Every connection has an ID that is unique among the set of currently connected clients.

mysql>
mysql> SELECT CONNECTION_ID();
+-----------------+
| CONNECTION_ID() |
+-----------------+
|              37 |
+-----------------+
1 row in set (0.00 sec)

mysql>