Add one new column and populate?

[Gordon Royle]

I have a mySQL table with about 12 million rows. Suppose I now compute one extra piece of data (a small integer) for each row, thus creating a file containing 12 million additional values that I wish to incorporate into my table. I know how to add an extra column to the table (ALTER) but I just cannot figure out how to populate the new column with the additional data WITHOUT deleting the old data? I guess that I can write 12 million UPDATE statements, but this seems pretty horrible.

Any clues much appreciated.

Gordon