Question

i have a table like below:

create table info (username varchar(30),otherinfo varchar(100));

now i want to alter this table to have new field and this field has to have default value as

md5(username)

something like below:

alter table info add NewField varchar(100) default md5(username);

how to do so?
Thanks for your help

Answer 1

Per MySQL docs (emphasis added) you cannot have an expression in a default value:

10.1.4. 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.

I’ve tested that the following trigger works for your intent:

CREATE TRIGGER MyTriggerName
  BEFORE INSERT ON info
  FOR EACH ROW
    SET NEW.NewField = md5(NEW.username);