fast update with 1:1 table?

[Mitchell Maltenfort]

I have two tables, A and B.  B has newer data, A has more columns.

I want to update some of the columns in A with all but one of the
columns in B.  They have the same number of records -- about eight
million -- and one key column in common for matching. (That's the one
from B I don't want to update)

I get the idea that using UPDATE...WHERE is not my fastest option.

Can anyone suggest an alternative?

Not sure about LOAD INFILE and REPLACE because they seem to cover entire rows.

Thanks in advance.