MERGE INTO DAT_DES as Target
USING DAT_STAGE as Source
ON Target.ds_nme=Source.ds_nme
 and Targer.lvl_nme = LTRIM(REPLACE(Source.lvl_nme, 'D', ''))
When NOT MATCHED BY TARGET THEN
INSERT (lvl_nme, ds_nme, ds_size, create_dt, rating_dt)
VALUES LTRIM(REPLACE(Source.lvl_nme, 'D', '')), Source.ds_nme, Source.ds_size, Source.create_dt, substring(Source.ds_nme, 3, 8));

insert into DAT_DES
(lvl_nme, ds_nme, ds_size, create_dt, rating_dt)
SELECT LTRIM(REPLACE(DAT_STAGE.lvl_nme, 'D', '')), ds_nme, ds_size, create_dt, substring(ds_nme, 3, 8)
from DAT_STAGE s
WHERE NOT EXISTS 
     (SELECT 1
      FROM DAT_DES d
      WHERE d.ds_nme=s.ds_nme
             AND d.lvl_nme=LTRIM(REPLACE(s.lvl_nme, 'D', '')))