MERGE INTO A1.TRG_TABLE TRG$
USING ( SELECT
   SRCT_ID AS TRGT_ID
  ,SRCT_NAME AS TRGT_NAME
  ,SRCT_DESC AS TRGT_DESC
FROM SRC_TABLE) SRC$
ON(
  TRG$.TRGT_ID=SRC$.TRGT_ID
)
WHEN MATCHED THEN UPDATE SET
   TRG$.TRGT_NAME=SRC$.TRGT_NAME
  ,TRG$.TRGT_DESC=SRC$.TRGT_DESC
WHERE --update when
      (TRG$.TRGT_NAME is null and SRC$.TRGT_NAME is not null)
   or (TRG$.TRGT_NAME is not null and SRC$.TRGT_NAME is null)
   or TRG$.TRGT_NAME!=SRC$.TRGT_NAME
   or (TRG$.TRGT_DESC is null and SRC$.TRGT_DESC is not null)
   or (TRG$.TRGT_DESC is not null and SRC$.TRGT_DESC is null)
   or TRG$.TRGT_DESC!=SRC$.TRGT_DESC
WHEN NOT MATCHED THEN INSERT(
   TRG$.TRGT_KEY
  ,TRG$.TRGT_ID
  ,TRG$.TRGT_NAME
  ,TRG$.TRGT_DESC
)VALUES(
   SEQ_TRG_TABLE.NEXTVAL
  ,SRC$.TRGT_ID
  ,SRC$.TRGT_NAME
  ,SRC$.TRGT_DESC
);