Remove duplicates using only where condition - sql

Question

Today, i got a problem from a friend.
Problem - Write a SQL query using UNION ALL(not union) that uses the where clause to eliminate duplicates.
I can not use group by expression
I can not use unique , distinct keywords.
Input -
id(Table 1)
1
2
fk_id(Table 2)
1
1
2
I gave him the solution below query
select id from
(
select id , row_number() over(partition by id order by id) rn from
(
select id from T1
union all
select fk_ID id from T2
)
)where rn = 1;
Output -
id
1
2
which is generating unique id's.
Now suspense by him i also can not use row_number(). i just have to use where condition. i am writing query on oracle database.
Please suggest.
Thanks in advance.

Answer 1

From its name and the data shown, we can assume that id in table t1 is unique.
From its name and the data shown, we can assume that fk_id in table t2 is a foreign key to table1.id.
So the union of the IDs in the two tables are simply the IDs that we find in table t1.
As we are forced to use UNION ALL on the two tables, though, we can use a pseudo UNION ALL not adding anything:
select id from t1
union all
select fk_id from t2 where 1 = 2;
If t2.fk_id were not a foreign key referencing t1.id, we would use NOT EXISTS or NOT IN in the where clause instead. If this is to give a result without duplicates, however, there must be no duplicates in t2 then to start with. (As you are showing that duplicate values in t2 do exist, this approach would not work then.) Here is a query for unique values from t1 plus unique values from t2 that are not referencing the t1 values:
select id from t1
union all
select fk_id from t2 where fk_id not in (select id from t1);

Answer 2

In a more generic case, where you can have duplicates in both tables, this could be a way.
test data:
create table table1(id) as (
select 1 from dual union all
select 1 from dual union all
select 2 from dual union all
select 2 from dual union all
select 1 from dual
)
create table table2(fk_id) as (
select 1 from dual union all
select 1 from dual union all
select 1 from dual union all
select 3 from dual union all
select 4 from dual union all
select 1 from dual union all
select 4 from dual union all
select 2 from dual
)
query:
with tab1_union_all_tab2 as (
select 'tab1'||rownum as uniqueId, id from table1 UNION ALL
select 'tab2'||rownum , fk_id from table2
)
select id
from tab1_union_all_tab2 u1
where not exists ( select 1
from tab1_union_all_tab2 u2
where u1.id = u2.id
and u1.uniqueId < u2.uniqueId
)
result:
ID
----------
3
4
1
2
This should clarify the idea behind:
with tab1_union_all_tab2 as (
select 'tab1'||rownum as uniqueId, id from table1 UNION ALL
select 'tab2'||rownum , fk_id from table2
)
select uniqueId, id,
( select nvl(listagg ( uniqueId, ', ') within group ( order by uniqueId), 'NO DUPLICATES')
from tab1_union_all_tab2 u2
where u1.id = u2.id
and u1.uniqueId < u2.uniqueId
) duplicates
from tab1_union_all_tab2 u1
UNIQUEID ID DUPLICATES
---------- ---------- --------------------------------------------------
tab11 1 tab12, tab15, tab21, tab22, tab23, tab26
tab12 1 tab15, tab21, tab22, tab23, tab26
tab13 2 tab14, tab28
tab14 2 tab28
tab15 1 tab21, tab22, tab23, tab26
tab21 1 tab22, tab23, tab26
tab22 1 tab23, tab26
tab23 1 tab26
tab24 3 NO DUPLICATES
tab25 4 tab27
tab26 1 NO DUPLICATES
tab27 4 NO DUPLICATES
tab28 2 NO DUPLICATES
As rightly observed by Thorsten Kettner, you can easily edit this to use rowid instead of building a unique id by concatenating a string and the rownum:
with tab1_union_all_tab2 as (
select rowid uniqueId, id from table1 UNION ALL
select rowid , fk_id from table2
)
select id
from tab1_union_all_tab2 u1
where not exists ( select 1
from tab1_union_all_tab2 u2
where u1.id = u2.id
and u1.uniqueId < u2.uniqueId
)

Answer 3

write a where statement for the second select in the union all as where id != fk_id