MERGE table, do nothing when matched

I have a table DOMAINS in 2 different schemas with columns ID, NAME,CODE,DESCRIPTION.

For any NAME exist in new schema, it should use existing ID without any merge; for those new NAME records, it should insert with ID from old schema.

MERGE INTO DOMAINS A
USING (SELECT ID,NAME,CODE,DESCRIPTION FROM <Old Schema 6.1>.DOMAINS@DB_MIG_61_TO_74) B
ON(A.NAME = B.NAME)
WHEN MATCHED **<do nothing>**
WHEN NOT MATCHED THEN INSERT(A.ID,A.NAME,A.CODE,A.DESCRIPTION) 
VALUES(B.ID,B.NAME,B.CODE,B.DESCRIPTION);

How can i intepret the portion of do nothing in above query?

Answer

For your case, no need to use the part:

WHEN MATCHED THEN UPDATE ...

( using WHEN MATCHED THEN UPDATE SET a.id = a.id is accepted(Oracle doesn’t hurl) but has no impact, so, such a usage is redundant, because you don’t want to change anything for the matching case. )

If you wanted to change, then add

WHEN MATCHED THEN UPDATE SET a.id = b.id

before WHEN NOT MATCHED THEN INSERT...

( e.g.Oracle supports WHEN MATCHED THEN UPDATE syntax. Refer the Demo below )

Go on with the following for the current case :

SQL> CREATE TABLE domains( 
                           id          INT, 
                           name        VARCHAR2(50), 
                           code        VARCHAR2(50), 
                           description VARCHAR2(50)
                         );

SQL> INSERT INTO domains VALUES(1,'Domain A','D.A.','This is Domain A');

SQL> MERGE INTO domains A USING 
     (SELECT 2 id, 'Domain A' name, 'D.A.' code, 'This is Domain A' description 
        FROM domains) b
          ON ( a.name = b.name )
        WHEN NOT MATCHED THEN INSERT( a.id, a.name, a.code, a.description ) 
                              VALUES( b.id, b.name, b.code, b.description );

SQL> SELECT * FROM domains;

ID  NAME        CODE    DESCRIPTION
--  --------   -----  ----------------
1   Domain A    D.A.  This is Domain A

SQL> DELETE domains;

SQL> INSERT INTO domains VALUES(1,'Domain A','D.A.','This is Domain A');
-- we're deleting and inserting the same row again

SQL> MERGE INTO domains A USING       
 (SELECT 2 id, 'Domain B' name, 'D.B.' code, 'This is Domain B' description 
    FROM domains) b
      ON ( a.name = b.name )
    WHEN NOT MATCHED THEN INSERT( a.id, a.name, a.code, a.description ) 
                          VALUES( b.id, b.name, b.code, b.description );


SQL> SELECT * FROM domains;

ID  NAME        CODE    DESCRIPTION
--  --------   -----  ----------------
1   Domain A    D.A.  This is Domain A
2   Domain B    D.B.  This is Domain B

Demo