Aggregate functions on multiple joined tables

Question

I have three tables:

CREATE TABLE foo (
    id bigint PRIMARY KEY,
    name text NOT NULL
);

CREATE TABLE foo_bar (
    id bigint PRIMARY KEY,
    foo_id bi         


        

Answer 1

SELECT f.id, f.name, b.fb_ct, t.tag_names
FROM   foo f
LEFT JOIN  (
    SELECT foo_id AS id, count(*) AS fb_ct
    FROM   foo_bar
    GROUP  BY 1
    ) b USING (id)
LEFT JOIN  (
    SELECT target_id AS id, array_agg(name) AS tag_names
    FROM   tag
    GROUP  BY 1
    ) t USING (id)
ORDER  BY f.id;

Produces the desired result.

• Rewrite with explicit JOIN syntax. Makes it so much easier to read and understand (and debug).

• By joining to multiple 1:n related tables, rows would multiply each other producing a Cartesian product - which is very expensive nonsense. It's an unintended CROSS JOIN by proxy. Related:

  • Two SQL LEFT JOINS produce incorrect result

• To avoid this, join at most one n-table to the 1-table before you aggregate (GROUP BY). You could aggregate two times, but it's cleaner and faster to aggregate n-tables separately before joining them to the 1-table.

• As opposed to your original (with implicit INNER JOIN). I use LEFT JOIN to avoid losing rows from foo that have no matching row in foo_bar or tag.

• Once the unintended CROSS JOIN is removed from the query, there is no need for adding DISTINCT any more - assuming that foo.id is unique.