update table column after insert new record using MySQL triggers

Question

Imagine I have a MySQL table (tbl_test) with these fields: id, title, priority.
id will be incremented automatically. I need to fill

Answer 1

The way you are trying to set value to a column is an update. Because you are doing it after insert operation is completed.

You actually need a before trigger.

And to assign the same new auto incremented value of primary key column of same table, you better get it from information_schema.tables.

Example:

delimiter //
drop trigger if exists bi_table_name //

create trigger bi_table_name before insert on table_name
for each row begin
  set @auto_id := ( SELECT AUTO_INCREMENT 
                    FROM INFORMATION_SCHEMA.TABLES
                    WHERE TABLE_NAME='table_name'
                      AND TABLE_SCHEMA=DATABASE() ); 
  set new.priority= @auto_id;
end;
//

delimiter ;

Note: Make sure that you don't have any pre-defined trigger with the same name and/or action. If have some, then drop them before creating the new.

Observations:
As per mysql documentation on last_insert_id(),

"if you insert multiple rows using a single INSERT statement, LAST_INSERT_ID() returns the value generated for the first inserted row only."

hence, depending on last_insert_id() and auto_increment field values in batch inserts seems not reliable.