Oracle - conditional merge on multiple columns

Question

I am running a merge on a single table with the following query:

    MERGE INTO tbl rs
    USING 
    SELECT res.sid, res.eid, res.a, res.b, res.c, res.d, res.e /* ... */
    /* Merge magic here */
    WHEN MATCHED THEN
        UPDATE SET rs.a = res.a, rs.b = res.b, rs.c = res.c, rs.d = res.d, rs.e = res.e;

Now, when one column contains different data than the original the update would be run on all 5 columns in a given row. Is there a way to restrict the update to update only the columns which are different and leave out the columns without differing data?

Answer 1

This is not exactly what you asked for, but you could restrict the UPDATE to those rows with at least one changed value with a WHERE clause in your UPDATE:

MERGE INTO tbl rs
USING 
  (SELECT res.sid, res.eid, res.a, res.b, res.c, res.d, res.e /* ... */ )
ON ( /* Merge magic here */ )
WHEN MATCHED THEN    
UPDATE SET 
      rs.a = res.a, 
      rs.b = res.b, 
      rs.c = res.c, 
      rs.d = res.d, 
      rs.e = res.e
WHERE rs.a <> res.a 
   OR rs.b <> res.b 
   OR rs.c <> res.c 
   OR rs.d <> res.d 
   OR rs.e <> res.e;

(depending on your data / application logic, you might want to add extra checking for NULL values).