It looks like you can't access the auto increment value directly using the before insert trigger. As part of your trigger you could try to select the current auto increment for the table, and then add one as that should be the next value.
Juacala - Web Application Developer, Eliacom, Inc. MySQL GUI Tools for Online Applications: Eliacom
Probably you are right I am not getting auto increment value directly. As you suggested to get current auto increment for table using trigger , but not show how will I get it? But I have got it from information_schema of mysql table.
guelphdad is right about the race condition problem. If you really want a concatenated column, the auto increment columns appear to be available if you do an AFTER INSERT trigger instead of a BEFORE INSERT. You could use that to update the row after it's been inserted. That should do the trick without the option of another row sneaking in while the trigger is pending.
I got auto_increment from information schema and doing well. But as suggested by guelphdad that there may be race condition occur then which will be better way to do it. BEFORE INSERT OR AFTER INSERT and why?