This syntax allows a comment to extend over multiple lines not be on the same line. : comments « Introduction « MySQL Tutorial

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mysql>
mysql>
mysql> SELECT 1 /* this is an in-line comment */ + 1;
+---------+
| 1   + 1 |
+---------+
|       2 |
+---------+
1 row in set (0.00 sec)

mysql> SELECT 1+
    -> /*
   /*> this is a
   /*> multiple-line comment
   /*> */
    -> 1;
+-------+
| 1+

1 |
+-------+
|     2 |
+-------+
1 row in set (0.00 sec)

mysql>