Three table join where middle table has duplicate foreign keys - sql

Question

I am working with a database with a structure similar to the illustration below (except with more columns). Basically, each person has a unique person_id and alt_id. However, the only thing connecting table A to table C is table B, and table B has one to many rows for each person/alt_id.
I need to get rows with a person_id, their alt id and their associated shapes.
I could do this:
SELECT DISTINCT a.person_id, a.color, b.alt_id, c.shape
FROM a
JOIN b ON a.person_id = b.person_id
JOIN c ON b.alt_id = c.alt_id
However, that seems inefficient as it will take a Cartesian product of rows from B and C with the same alt_id before finally using DISTINCT to narrow the results down. What's the best/most efficient way to do this query?
Table A
+-----------+-------+
| person_id | color |
+-----------+-------+
| 10 | red |
| 11 | blue |
| 12 | green |
+-----------+-------+
Table B
+-----------+--------+
| person_id | alt_id |
+-----------+--------+
| 10 | 225 |
| 10 | 225 |
| 11 | 226 |
| 11 | 226 |
| 11 | 226 |
| 12 | 227 |
+-----------+--------+
Table C
+--------+----------+
| alt_id | shape |
+--------+----------+
| 225 | square |
| 226 | circle |
| 226 | rhombus |
| 226 | ellipse |
| 227 | triangle |
+--------+----------+

Answer 1

Join to (select distinct * from b) b rather than just the base table b.
SELECT
a.person_id, a.color, b.alt_id, c.shape
FROM
a
INNER JOIN (select distinct * from b) b
ON a.person_id = b.person_id
INNER JOIN c
ON b.alt_id = c.alt_id

Answer 2

You can get a distinct list of values from b before you do your joins.
SELECT DISTINCT a.person_id, a.color, b.alt_id, c.shape
FROM a
JOIN (Select Distinct person_id, alt_id from b) b ON a.person_id = b.person_id
JOIN c ON b.alt_id = c.alt_id
Note that because of indexes, and statistics, getting a DISTINCT list is not always a good idea. Look at the actual execution plan to evaluate how good this is, especially if you have a lot of data.

Answer 3

You could use aggregation along with a common table expression (or subquery, but a CTE might be neater):
WITH ab AS (
SELECT a.person_id, a.color, MAX(b.alt_id) AS alt_id
FROM a INNER JOIN b
ON a.person_id = b.person_id
GROUP BY a.person_id, a.color
)
SELECT ab.person_id, ab.color, ab.alt_id, c.shape
FROM ab INNER JOIN c ON ab.alt_id = c.alt_id;