Update vs Merge - Updating one table based on values in another

Question

Hello
I have two tables lets say TAB_A and TAB_B. I altered table B to include a new column from table A
I wrote a merge statement as follows to merge the data
MERGE INTO TAB_A
USING TAB_B
ON (TAB_A.SECURITYPERSONKEY=TAB_B.SECURITYPERSONKEY)
WHEN MATCHED THEN
  UPDATE SET TAB_A.PPYACCOUNT=TAB_B.PPYACCOUNT;
I know INSERT is for inserting new records
UPDATE to my knowledge is to modify currently existing records (loosely)
MERGE is one I rarely used, until this particular scenario.
The code works perfectly fine, but I was wondering how could I write an update statement? Or in this scenario should I even be using an update statement?

Answer 1

The MERGE is good and performs well, although it malfunctions (sometimes updating data wrongly) if the destination is a view with INSTEAD OF triggers.
The UPDATE with similar performance to the MERGE depends on SECURITYPERSONKEY being a unique or primary key on TAB_B. It is:
update (select TAB_A.PPYACCOUNT a_PPYACCOUNT, TAB_B.PPYACCOUNT b_PPYACCOUNT
        from TAB_A JOIN TAB_B on TAB_A.SECURITYPERSONKEY=TAB_B.SECURITYPERSONKEY)
SET A_PPYACCOUNT=B_PPYACCOUNT;
This can set multiple columns. It also currently doesn't work on a view with INSTEAD OF triggers.
Frank's update also works with multiple columns (eg also updating col2 and col2), with this syntax:
UPDATE  tab_a
SET     (ppyaccount, col2, col3) = (
                         SELECT  ppyaccount, col2, col3
                         FROM   tab_b
                         WHERE  securitypersonkey  = tab_a.securitypersonkey
WHERE   EXISTS       (
                         SELECT  ppyaccount
                         FROM    tab_b
                         WHERE   securitypersonkey  = tab_a.securitypersonkey