Question

I need to create a field in a database table, already populated with data. Of course just adding the field empty is not possible. I can only think of creating a new table with a new structure and copy the existing data to the new table, but I wonder if there is a n easier way.

Hint: it is a composite key, comprised of 3 other fields in the same table.

Edit: the field holds a varchar value
Edit: since some of you ask, it is not possible to create a new UNIQUE field in a populated table just by using ADD UNIQUE. It duplicates the new (empty or not) value throughout all entries. Example:

ALTER TABLE ‘tablename’ ADD ‘fieldname’ VARCHAR( 64 ) NOT NULL ,
ADD UNIQUE (
‘fieldname’
)

error: Duplicate entry ” for key ‘fieldname’

Answer 1

Of course just adding the field empty is not possible.

Why?

I’d do the following:

1. Create field by ALTER TABLE t ADD COLUMN new_column *type_definition*
2. Update newly created field like UPDATE t SET new_column=*computation_expression*
3. Add index by ALTER TABLE t ADD INDEX ... (or ALTER TABLE t ADD PRIMARY KEY ... if you need it to be primary).