str1 corresponds to bit 0, str2 to bit 1, and so on.
NULL values in str1, str2, ... are not appended to the result.
mysql>
mysql> SELECT MAKE_SET(1,'a','b','c');
+-------------------------+
| MAKE_SET(1,'a','b','c') |
+-------------------------+
| a |
+-------------------------+
1 row in set (0.00 sec)
mysql>