Question

I am comparing two tables A and B. A and B are months (we will go with JAN and FEB).
FEB has updated data that belongs in JAN.

I need to update the data like so

UPDATE A
SET A.x = B.x, A.y = B.y, A.z = B.z
FROM JAN A, FEB B
WHERE (A.x <> B.x OR A.y <> B.y OR A.z <> B.z) AND A.PK = B.PK

Now I want the above to not take place on the Original JAN table.
Should I go about it this way? Or is there a better way?

SELECT *
INTO JAN_UPDATED
FROM JAN

UPDATE A
SET A.x = B.x, A.y = B.y, A.z = B.z
FROM JAN_UPDATED A, FEB B
WHERE (A.x <> B.x OR A.y <> B.y OR A.z <> B.z) AND A.PK = B.PK

EDIT: I want all of the original values + the updates in the new table

EDIT: Added PK

Answer 1

Using an outer join, you could select the data into a new table and update them along the way. Here’s how:

SELECT    A.PK,
          COALESCE(B.x, A.x) AS x,
          COALESCE(B.y, A.y) AS x,
          COALESCE(B.z, A.z) AS x,
          other columns as necessary
INTO      JAN_UPDATED
FROM      JAN A
LEFT JOIN FEB B ON A.PK = B.PK AND (A.x <> B.x OR A.y <> B.y OR A.z <> B.z)

The left part of the join, the JAN table, will return all the JAN rows, and the right part of the join, FEB, will only return matching rows, those which are different from their counterparts in JAN. When there’s no match, the right part’s columns will be filled with NULLs. Now, when pulling values, the COALESCE() function is used for x, y, and z: the FEB version is tried first, and if it is NULL (meaning this is a non-matching JAN row, the one that has no updates in FEB), then the JAN party (the unchanged value) is used instead.