I have two databases with identical structures, but different data.

I want to make DatabaseB's TableABC exactly the same as DatabaseA's
TableABC, with one exception: If any particular key (FieldABC) previously
exists in DatabaseB's version of the table, I want to preserve the value in
FieldXYZ. I also want to get rid of any keys in DatabaseB's version of the
table that don't exist in DatabaseA's version.

Example: (each grouping of 3 fields is a row in TableABC)

DatatbaseA
================

FieldABC = 123
FieldDEF = 456
FieldXYX = 789

FieldABC = 1234
FieldDEF = 4567
FieldXYX = 7890

FieldABC = 12345
FieldDEF = 45678
FieldXYX = 78901



Database B (BEFORE)
==================

FieldABC = 1234
FieldDEF = 9999
FieldXYX = 10000

FieldABC = 9999
FieldDEF = 4567
FieldXYX = 10001

Database B (AFTER)
================

FieldABC = 123
FieldDEF = 456
FieldXYX = 789

FieldABC = 1234
FieldDEF = 4567
FieldXYX = 10000

FieldABC = 12345
FieldDEF = 45678
FieldXYX = 78901

As you can see, DatabaseB's version of the table becomes exactly the same
as DatabaseA's, except that in the one row that already existed, DatabaseB,
FieldXYZ retained it's old value of 10000. FieldDEF got DatabaseA's value
for that key. Also key 9999 got removed from DatabaseB because it did not
exist in DatabaseA.

How could I achieve this result?

My first thought was to loop through all the rows in DatabaseA, querying
each key in DatabaseB to see if it exists. If it doesn't, insert a new row
with DatabaseA's values. If it does exist, update all the fields in
DatabaseB's row to match DatabaseA's row, except for FieldXYZ. The only
problem with this is that it doesn't do anything about getting rid of old
rows in DatabaseB that didn't exist in DatabaseA. I can't do a simple
delete with an existence test in the Where clause, because we're working
with two separate databases. I suppose when it's all done, I could loop
through all of DatabaseB's rows and query for each key in DatabaseA, and if
it doesn't exist, delete it from DatabaseB. It seems like a lot of steps,
though.

My second thought was to just delete everything in DatabaseB's table, and
import it in from DatabaseA. The only problem is how do I preserve the
values for FieldXYZ in Database B? I was thinking of just storing a linked
list in the software of DatabaseB's original values of FieldXYZ for each
key, then updating them when it's all done. But, if something goes wrong
(program hangs, user aborts, network connection goes down, etc.), Database
B's original values are lost.

Does anyone have a more elegant solution that I haven't thought of?

-Joe