How to achieve the following logic

Question

I have a target and a source table.
CREATE TABLE scd_tgt(
col1 VARCHAR2(10) not null,
col2 VARCHAR2(10) not null,
start_date DATE not null,
end_date DATEnot null
INSERT INTO scd_tgt VALUES('100','AKS','1-JAN-2010','9-SEP-9999');
INSERT INTO scd_tgt VALUES('101','Singh','1-JAN-2010','9-SEP-9999');
CREATE TABLE scd_src(
col1 VARCHAR2(10) not null,
col2 VARCHAR2(10) not null
INSERT INTO scd_src VALUES('99','Kumar');
INSERT INTO scd_src VALUES('100','Abhijit');
INSERT INTO scd_src VALUES('101','Singh');
Now, I have to perform update/insert data in scd_tgt using source data in scd_src based on the following rules:
1) INSERT any records from the SOURCE that aren't already in the TARGET, based on a col1.
2) If the TARGET has a record present in SOURCE (based on col1) and col2 of TARGET does not match with col2 of SOURCE then UPDATE the end_date to SYSDATE - 1. Proceed to Step 3.
3) INSERT the record from #2 into TARGET. Set start_Date as SYSDATE and end_date as '9-sep-9999'
Can this be achieved using MERGE? I am using 10.2g
After executing the steps scd_tgt should have the following data
99     Kumar     18-JUN-10     09-SEP-99
100     AKS     01-JAN-10     17-JUN-10
100     Abhijit     18-JUN-10     09-SEP-99
101 Singh 01-JAN-10     09-SEP-99

Answer 1

Hi,
With a merge it's not possible if you want to to update and insert (or insert/update) at the same time within the same condition (when matched or when not matched parts of the stement), what's authorizzed with a merge is:
an update + a delete in the "when matched" clause
an insert in the "when not matched" clause
So one solution could be that you use two differents DML op:
UPDATE scd_tgt
   SET end_date =   TRUNC(SYSDATE)
                  - 1
WHERE col1 IN(SELECT col1
               FROM   scd_src)
INSERT INTO scd_tgt
   SELECT col1,
          col2,
          TRUNC(SYSDATE),
          TO_DATE('09-sep-9999', 'dd-mon-yyyy')
   FROM   scd_srcIf the number of rows generally updated by the 1st statement is low (comparing to the volume of the scd_tgt table) an index on col1 in(scd_tgt table) would probably be benifical...
If you never delete from scd_tgt and if the performance goal is qiete important, then consider an /*+ APPEND */ hint with the insert ..
(depends on how many rows are inserted each time this code is ran).
As often this is not the only one solution but this the most simple one
-> In development always remind this addage: the most is simple the code is the better is (for everyone!) ..