Returns the IDs of documents that were inserted or replaced by the last statement in the current session.

The same value can also be obtained via the @@session.last_insert_id variable.

mysql> select @@session.last_insert_id;
+--------------------------+
| @@session.last_insert_id |
+--------------------------+
| 11,32                    |
+--------------------------+
1 rows in set

mysql> select LAST_INSERT_ID();
+------------------+
| LAST_INSERT_ID() |
+------------------+
| 25,26,29         |
+------------------+
1 rows in set   

Returns the current connection ID.

mysql> select CONNECTION_ID();
+-----------------+
| CONNECTION_ID() |
+-----------------+
| 6               |
+-----------------+
1 row in set (0.00 sec)