Oracle SQL: How to replace distinct with where exists - sql

Question

I've read that using a where exists clause could usually be more efficient than writing select distinct. How could I rewrite the below 2 queries using the where exists condition? Not sure if query2 is eligible for this clause or this only applies to joins.
Query 1:
SELECT DISTINCT
e.field1,
regexp_substr(substr(TRIM(d.field2), 1, 2), '[A-Za-z]+', 1, 1) postal_group
FROM
table1 e
JOIN table1 f ON f.field0 = e.field0
JOIN table2 g ON g.field3 = f.field3
JOIN table3 g ON g.field4 = f.field4
JOIN table4 a ON a.field5 = g.field5
JOIN table5 b ON ( b.field6 = a.field6
AND b.field7 = a.field7 )
JOIN table6 c ON ( c.field8 = b.field8
AND c.field9 = b.field9 )
JOIN table7 d ON ( d.field10 = c.field10
AND d.field11 = c.field11 )
Query 2:
SELECT DISTINCT
field
FROM
table1
WHERE
condition = 'value'

Answer 1

For your second query you can use group by clause to avoid DISTINCT.
SELECT field
FROM
table1
WHERE
condition = 'value'
Group by field
Please try below query with where exists instead of Distinct:
SELECT
e.field1,
regexp_substr(substr(TRIM(d.field2), 1, 2), '[A-Za-z]+', 1, 1) postal_group
FROM
table1 e
where exists
(
select 1 from table1 f
JOIN table2 g ON g.field3 = f.field3
JOIN table3 g ON g.field4 = f.field4
JOIN table4 a ON a.field5 = g.field5
JOIN table5 b ON ( b.field6 = a.field6
AND b.field7 = a.field7 )
JOIN table6 c ON ( c.field8 = b.field8
AND c.field9 = b.field9 )
JOIN table7 d ON ( d.field10 = c.field10
AND d.field11 = c.field11 )
where f.field0 = e.field0
)

Answer 2

You can't avoid a join with d because you need a column from it. Therefore the best you can do is:
select e.column1
, d.column2 as postal_group
from table1 e
join table7 d
on d.column10 = c.column10
and d.column11 = c.column11
where exists
( select 1 from table1 f
join table2 g on g.column3 = f.column3
join table3 h on h.column4 = f.column4
join table4 a on a.column5 = h.column5
join table5 b on b.column6 = a.column6 and b.column7 = a.column7
join table6 c on c.column8 = b.column8 and c.column9 = b.column9
where f.column0 = e.column0 );
You only need distinct if table7.column2 can have duplicate values for a (column10, column11) combination.
This may or may not be more efficient than the original version - compare timings, execution plans, reads etc.
(I've renamed 'fields' as columns because that's what tables have. Also there are no brackets in a join clause and adding them tends to confuse code formatters.)