mysql set field default value to other column

Question

How to set default value for a field to other column in Mysql I have done it oracle with virtual field but I do not know how to do it in Mysql this is my table:

create table TSM_TRANSACTION_TBL
(
  TRANS_ID     INT primary key auto_increment,
  LOCATION_ID  INT,
  TRANS_DATE   DATE,
  RESOURCE_ID  INT,
  TS_ID        INT,
  MAX_VALUE    INT,
  BOOKED_UNITS INT default 0,
  REMAINING    INT default MAX_VALUE - BOOKED_UNITS,
  BOOKED       INT not null,
  USER_ID      INT,
  TRANS_TIME   TIMESTAMP
)

Answer 1

As documented under Data Type Default Values:

The DEFAULT value clause in a data type specification indicates a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression. This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column. See Section 11.3.5, “Automatic Initialization and Updating for TIMESTAMP”.

Instead, you can define an insertion trigger:

CREATE TRIGGER foo BEFORE INSERT ON TSM_TRANSACTION_TBL FOR EACH ROW
  IF NEW.REMAINING IS NULL THEN
    SET NEW.REMAINING := NEW.MAX_VALUE - NEW.BOOKED_UNITS;
  END IF;;

Answer 2

The 'NEW' is unacceptable for AFTER insert triggers. You should do the 'field update' by a BEFORE insert trigger. So,

CREATE TRIGGER foo BEFORE INSERT ON TSM_TRANSACTION_TBL FOR EACH ROW
    IF NEW.REMAINING IS NULL THEN
        SET NEW.REMAINING := NEW.MAX_VALUE - NEW.BOOKED_UNITS;
    END IF;;