MYSQL set default value of column to be new id

unique_id | master_id | othercolumns

If unique_id is a 开发者_如何学Ccolumn with auto increment, is there a way to have the column master_id default value be the unique_id when inserting a new record?

I take it "unique_id" and "master_id" are columns. No, you can't have one column default to the value of another, but you can create a trigger to set master_id to the value of unique_id on insert. Say the default for master_id is 0. Then the following trigger should work:

delimiter ;;

CREATE TRIGGER default_master_id
  BEFORE INSERT
  ON table_name FOR EACH ROW
  IF NEW.master_id = 0 THEN
    SET NEW.master_id = last_insert_id()+1 ;
  END IF ;;

delimiter ;

insert into table ... set master_id=last_insert_id()+1;