delete from tab_a where tab_a.primary_key in 
  (select primary_key from tab_a as a
   where exists (select 1 from tab_b as b 
        where b.column1 in (case when a.column1 = '' then substr(column2,1,1) else a.column1 end) and a.column3 = b.column3 and b.column4 >= a.column4 and a.column5 = ' ' and a.column6 <> b.column6 and b.column7 > '2010-09-01' )
   fetch first 1000 rows only)

Delete from tab_a  A
Where a.column5 = ' ' and
exists 
(select 1 from tab_b B where 
b.column1 in 
(case when a.column1 = ' ' 
then substr(column2,1,1) 
else a.column1 
end) 
and a.column3 = b.column3 
and b.column4 >= a.column4  
and a.column6 <> b.column6 
and b.column7 > '2010-09-01' )