Getting duplicates when joining on the same table twice and another table once (3 total)

Question

I'm relatively new to sql and have been trying to get a pretty complex query (for me) to work for a while, but I've kept on getting duplicate values within each column using node postgres. Through this query I am trying to track user info, plan info and email info on a dashboard. Before we get to the query, here are the tables -

USER TABLE (u) - keep track of user info

+----+-------+---------+-------------+----------+ | id | first | last | email | password | +----+-------+---------+-------------+----------+ | 1 | joe | smith | [email protected] | 1234 | | 2 | mary | johnson | [email protected] | 3445 | | 3 | harry | gold | [email protected] | 4345 | +----+-------+---------+-------------+----------+

PLANS TABLE (p) - plans that users can make with their friends

+----+--------------+-----------+------------+------+--------+-----------+---------+------+ | id | experienceid | hostid(u) | guestid(u) | date | time | paidid(u) | groupid | newp | +----+--------------+-----------+------------+------+--------+-----------+---------+------+ | 33 | 1 | 1 | [1,2,3] | 4/20 | 8:00pm | [1,2] | 1 | true | +----+--------------+-----------+------------+------+--------+-----------+---------+------+

EMAIL TABLE (e) - keep track of messages I am sending to the users based on the plan they are a part of

+-------------+-----------+---------+----------+ | email(u) | planid(p) | confirm | reminder | +-------------+-----------+---------+----------+ | [email protected] | 33 | null | null | | [email protected] | 33 | true | false | | [email protected] | 33 | true | false | +-------------+-----------+---------+----------+

Now for the query what I am trying to do is combine all three tables to get this output -

+-------+---------------+---------------------------+---------+---------+------------+---------------+---------------+ | id(p) | hostname(u+p) | paidguests(u+p) | time(p) | newp(p) | groupid(p) | reminder(e) | confirm(e) | +-------+---------------+---------------------------+---------+---------+------------+---------------+---------------+ | 33 | joe smith | [joe smith, mary johnson] | 8:00pm | true | 1 | [true, false] | [true, false] | +-------+---------------+---------------------------+---------+---------+------------+---------------+---------------+

Now where I left off in the query, I almost got it to work, but I kept getting duplicate values where it would look like this -

+-------+---------------+----------------------------------------------------+---------+---------+------------+----------------------------+---------------------------+ | id(p) | hostname(u+p) | paidguests(u+p) | time(p) | newp(p) | groupid(p) | reminder(e) | confirm(e) | +-------+---------------+----------------------------------------------------+---------+---------+------------+----------------------------+---------------------------+ | 33 | joe smith | [joe smith, mary johnson, joe smith, mary johnson] | 8:00pm | true | 1 | [true, false, true, false] | [true, false, true false] | +-------+---------------+----------------------------------------------------+---------+---------+------------+----------------------------+---------------------------+

Now I don't really care about order for the confirm and reminder columns relative to the paidguests(u+p) column, as long as the right data is in there and not duplicated. Here is the query I had as it stands -

SELECT p.id, Concat(u.first, ' ', u.last) AS hostname, Array_agg(Concat(us.first, ' ', us.last)) AS paidguests, p.time, p.groupid, p.newp, Array_agg(e.confirm) AS confirm, Array_agg(e.reminder) AS reminder FROM plans p CROSS JOIN Unnest(p.paidid) AS allguests LEFT JOIN users us ON allguests = us.id LEFT JOIN emails e ON p.id = e.planid LEFT JOIN users u ON p.hostid = u.id WHERE p.experienceid = $1 AND p.date = $2 GROUP BY p.id, u.first, u.last, p.paidid, p.time, p.groupid, p.newp, confirm, reminder ORDER BY Array_length(p.paidid, 1) DESC

So essentially just looking to get the table right without the duplicates. It was working before I added the join to the email table, but not entirely sure why its duplicating.

Hope I was thorough in the explanation. If not, please let me know what I can clarify! Thanks so much :)

CREATE TABLE statements showing data types and constraints would be more instructive. And always your version of Postgres. — Erwin Brandstetter
– Erwin Brandstetter, Commented Apr 21, 2018 at 15:02
I will keep that in mind for next time - thanks for the heads up! — hasaab
– hasaab, Commented Apr 21, 2018 at 16:03

Rahul Jain · Accepted Answer · 2018-04-21 05:46:35Z

3

try adding this condition in where clause:

AND us.email = e.email

Culprit here is -> all emails are joined with all the users irrespective of their email id since all users and all emails have same plan_id. Hence the duplication.

answered Apr 21, 2018 at 5:46

Rahul Jain

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2 Comments

hasaab Over a year ago

Thank you that did it! Appreciate the help :)

coretechie Over a year ago

This helped me fix a problem with a similar issue.

Erwin Brandstetter · Accepted Answer · 2024-03-18 22:21:30Z

Rahul spotted the missing join condition. But the rabbit hole goes deeper. I suggest this query:

SELECT p.id , concat_ws(' ', u.first, u.last) AS hostname -- concat_ws! , p.time , p.groupid , p.newp , paid.paidguests , paid.confirm , paid.reminder FROM plans p LEFT JOIN users u ON u.id = p.hostid LEFT JOIN LATERAL ( -- LATERAL join SELECT array_agg(sub.paidguest) AS paidguests , array_agg(sub.confirm) AS confirm , array_agg(sub.reminder) AS reminder FROM ( SELECT concat_ws(' ', us.first, us.last) AS paidguest, e.confirm, e.reminder FROM unnest(p.paidid) WITH ORDINALITY AS paid(id, ord) JOIN users us ON us.id = paid.id LEFT JOIN emails e ON e.email = us.email AND e.planid = p.planid ORDER BY paid.ord ) sub ) paid ON true WHERE p.experienceid = $1 AND p.date = $2 -- no GROUP BY needed ORDER BY cardinality(p.paidid) DESC, p.id;

Assuming that (planid, email) is the PRIMARY KEY of the email table and there is a FOREIGN KEY constraint from email.email to users.email.

Aggregate first, then join, so you don't need to GROUP BY on all the columns that don't need aggregation. While retrieving all or most rows, other query techniques are typically faster, for a small selection like in your example, I suggest a LATERAL join. Related:

What is the difference between a LATERAL JOIN and a subquery in PostgreSQL?

In this particular case JOIN LATERAL would do the same as LEFT JOIN LATERAL since a subuery with aggregates and no GROUP BY always returns exactly 1 row.

The alias in your original Unnest(p.paidid) AS allguests is actively confusing, since those seem to be the IDs of guests who paid, not of all guests.

Use concat_ws() if first or last can be null. See:

How to concatenate columns in a Postgres SELECT?

When unnesting arrays, the order of elements is typically preserved in the simple case. But you have additional joins, so use WITH ORDINALITY and ORDER BY to avoid surprises. Your query may seem to work alright, even for a prolonged period of time - and then "suddenly" break (wrong order of elements) if you are not explicit about this. See:

PostgreSQL unnest() with element number

The whole DB design is debatable. Typically, these arrays should rather be implemented as related tables - for many reasons, beyond the scope of this question.

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Getting duplicates when joining on the same table twice and another table once (3 total)

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