Oracle 18c

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select * from V$VERSION;


BANNER
BANNER_FULL
BANNER_LEGACY
CON_ID


Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Version 18.4.0.0.0
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
0


--Результат подзапроса (отфильтрованных клиентов)
create table fq (idclient, idblank, datenter) as
    select 1, 1, date'2021-06-03'+0.5 from dual union all  
    select 5, 4, date'2021-03-13'+0.5 from dual union all 
    select 2, 1, date'2021-04-03'+0.5 from dual
    
3 rows affected

--Вся таблица
create table t (idclient, idblank, datenter) as
    select 5, 13, date'2021-01-14'+0.5 from dual union all
    select 1, 1, date'2021-06-03'+0.5 from dual union all  
    select 1, 5, date'2021-02-13'+0.5 from dual union all 
    select 2, 14, date'2021-06-03'+0.5 from dual union all  
    select 3, 12, date'2021-02-17'+0.5 from dual union all 
    select 4, 9, date'2021-04-03'+0.5 from dual union all  
    select 5, 6, date'2021-01-12'+0.5 from dual union all 
    select 6, 7, date'2021-04-03'+0.5 from dual union all  
    select 5, 4, date'2021-01-13'+0.5 from dual union all 
    select 3, 3, date'2021-04-03'+0.5 from dual union all  
    select 4, 2, date'2021-02-13'+0.5 from dual union all 
    select 2, 1, date'2021-04-03'+0.5 from dual union all
    select 1, 10, date'2021-05-25'+0.5 from dual
13 rows affected

select 
    fq.*, case 
          when add_months (fq.datenter, -3) <= prevFormIssued 
          then 'y' else 'n' end existLessAs3MonthOld
from fq
cross apply (
    select max (datenter) keep (dense_rank first order by datenter desc) prevFormIssued
    from t 
    where t.idclient = fq.idclient 
    and t.datenter < fq.datenter)


IDCLIENT
IDBLANK
DATENTER
EXISTLESSAS3MONTHOLD


1
1
03-JUN-21
y

5
4
13-MAR-21
y

2
1
03-APR-21
n