Syntax:
CONNECTION_ID()

Returns the connection ID (thread ID) for the connection. Every
connection has an ID that is unique among the set of currently
connected clients.

The value returned by CONNECTION_ID() is the same type of value as
displayed in the ID column of the INFORMATION_SCHEMA.PROCESSLIST table,
the Id column of SHOW PROCESSLIST output, and the PROCESSLIST_ID column
of the Performance Schema threads table.

URL: http://dev.mysql.com/doc/refman/5.5/en/information-functions.html

