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?

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

Btw, the part followed by the USING keyword no need to be a subquery, but a table or a view. Having evaluated for the current case:

MERGE INTO domains ds       --> "ds" : "domains" source
     USING v_domains dt     --> "dt" : "domains" target
        ON ( ds.name = dt.name )
      WHEN NOT MATCHED THEN INSERT( ds.id, ds.name, ds.code, ds.description ) 
                            VALUES( dt.id, dt.name, dt.code, dt.description )

might identically be used like the above statement after having created v_domains view through

CREATE OR REPLACE VIEW v_domains AS
SELECT 2 id, 'Domain A' name, 'D.A.' code, 'This is Domain A' description 
  FROM domains