Best way to eliminate duplicates rows after multiple joins - sql

Question

I'll consider three simple tables. A, B are my entity tables and C is an intermediate table that creates a many-to-many relationship between A & B.
Schemas:
A: (id INTEGER PRIMARY KEY)
B: (id INTEGER PRIMARY KEY)
C: (
A_id INTEGER,
B_id INTEGER,
FOREIGN KEY(A_id) REFERENCES A(id),
FOREIGN KEY(B_id) REFERENCES B(id)
)
Now, consider the below query
SELECT
A.id
FROM A
LEFT OUTER JOIN C
ON (A.id = C.A_id)
LEFT OUTER JOIN B
ON (C.B_id = B.id)
WHERE ...;
This query would result in duplicate values of A.id, which is expected because C might have multiple rows associated with each row of A. My question is what's the best way to eliminate these duplicates and get the A records. I only need the A records.
I am aware of two ways,
-- Using DISTINCT
SELECT
DISTINCT(A.id), ...
FROM A
LEFT OUTER JOIN C
ON (A.id = C.A_id)
LEFT OUTER JOIN B
ON (C.B_id = B.id)
WHERE ...
ORDER BY A.id;
And
-- Or using A.id IN (above query)/ A.id = Any(above query)
SELECT
...
FROM A
WHERE A.id IN (
SELECT
A.id
FROM A
LEFT OUTER JOIN C
ON (A.id = C.A_id)
LEFT OUTER JOIN B
ON (C.B_id = B.id)
WHERE ...
);
I'm using PostgreSQL. I need to include all the tables for filtering, so not joining a table cannot be considered as an improvement. I've analyzed both the queries but I still feel there might be a better way to do this(in terms of performance).
Any help is really appreciated!

Answer 1

I would suggest exists:
SELECT A.id
FROM A
WHERE EXISTS (SELECT 1
FROM C JOIN
B
ON C.B_id = B.id
WHERE A.id = C.A_id AND . . .
)

Answer 2

You can also try following query:
SELECT
a.* -- or whatever columns you need of a
FROM a
WHERE EXISTS(
SELECT 1
FROM c
WHERE c.a_id = a.id
)
Note, that there is no need to join table b as the existence of the row in c always guarantees for the row in b and you do not need any information contained in this row/table.
Perhaps even more clean might be:
SELECT DISTINCT
a.* -- or whatever columns you need of a
FROM a
LEFT JOIN c
You can have a look at the query plans and execution times using EXPLAIN ANALYZE <query>. Perhaps this gives you a hint on what to use best.
But be aware of caching, repeat both queries multiple times this way to see comparable results.