SQL Server 2022

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create table zmtarif
(
  firma int, 
  zmgnr varchar(3), 
  tahdnr varchar(2),
  formel varchar(2)
);
INSERT INTO zmtarif (firma, zmgnr, tahdnr, formel)
select 1, '001', '09', '99'
where not exists (select * 
                  from zmtarif t2
                  where t2.tahdnr = '09');
1 rows affected

select *
from zmtarif


firma
zmgnr
tahdnr
formel


1
001
09
99


INSERT INTO zmtarif (firma, zmgnr, tahdnr, formel)
select 1, '001', '09', '99'
where not exists (select * 
                  from zmtarif t2
                  where t2.tahdnr = '09');
select *
from zmtarif


firma
zmgnr
tahdnr
formel


1
001
09
99