Postgres: what's wrong with the syntax of this query? - sql

Question

I'm trying to write a fairly straightforward PSQL query to retrieve some data (I realise it's not the most efficient query right now):
SELECT c.name AS article, c.id AS article_id, t.name AS template, t.id AS template_id, brand_names, COUNT(p.component_id)
FROM publications p
INNER JOIN components c
(SELECT string_agg(b.name, ', ') AS brand_names
FROM brands b
INNER JOIN brands_components
ON b.id = brands_components.brand_id
WHERE brands_components.component_id = c.id
) brand_query
ON c.id = p.component_id
INNER JOIN brands_components bc
ON c.id = bc.component_id
AND bc.brand_id IN (16, 23, 24, 35, 37)
INNER JOIN components_templates ct
ON c.id = ct.component_id
INNER JOIN templates t
ON t.id = ct.template_id
This gives me a syntax error though on line 4. What's missing? If I run the subquery alone it works fine:
syntax error at or near "SELECT" LINE 4: (SELECT string_agg(b.name, ', ') AS brand_names ^ : SELECT c.name AS article, c.id AS article_id, t.name AS template, t.id AS template_id, brand_nam
The subquery is designed to retrieve all the brand names per component and display them in a single row instead of many. Their join table is brands_components.
A fiddle that is available here, the desired result should be something like:
article article_id template template_id count brands
--------------------------------------------------------------------------------------------------------------
component one | 1 | template one | 1 | 4 | brand one, brand two, brand three, brand four

Answer 1

Your immediate problem could be solved with a a lateral join:
SELECT c.name AS article, c.id AS article_id, t.name AS template, t.id AS template_id, brand_names, COUNT(p.component_id)
FROM publications p
JOIN components c ON c.id = p.component_id
JOIN brands_components bc ON c.id = bc.component_id AND bc.brand_id IN (1, 2, 3, 4)
JOIN LATERAL (
SELECT b.id, string_agg(b.name, ', ') AS brand_names
FROM brands b
JOIN brands_components ON b.id = brands_components.brand_id
WHERE brands_components.component_id = c.id
GROUP BY b.id
) brand_query ON brand_query.id = bc.brand_id
JOIN components_templates ct ON c.id = ct.component_id
JOIN templates t ON t.id = ct.template_id
GROUP BY 1,2,3,4
The above would still not run because the group by doesn't include the brand_names column. Postgres doesn't know that brand_names is already aggregates.
However, the derived table is not really needed if you move the aggregation to the outer query:
SELECT c.name AS article,
c.id AS article_id,
t.name AS template,
t.id AS template_id,
string_agg(b.name, ',') as brand_names,
COUNT(p.component_id)
FROM publications p
JOIN components c ON c.id = p.component_id
JOIN brands_components bc ON c.id = bc.component_id AND bc.brand_id IN (1, 2, 3, 4)
JOIN brands b on b.id = bc.brand_id
JOIN components_templates ct ON c.id = ct.component_id
JOIN templates t ON t.id = ct.template_id
GROUP BY c.name, c.id, t.name, t.id;

Answer 2

Try this Query:
SELECT c.name AS article, c.id AS article_id, t.name AS template, t.id AS template_id,MAX(brand_names) AS brand_names, COUNT(p.component_id) AS Counts
FROM publications p
INNER JOIN components c
ON c.id = p.component_id
INNER JOIN brands_components bc
ON c.id = bc.component_id
AND bc.brand_id IN (1, 2, 3, 4)
INNER JOIN components_templates ct
ON c.id = ct.component_id
INNER JOIN templates t
ON t.id = ct.template_id
INNER JOIN (SELECT string_agg(b.name, ', ') AS brand_names
FROM brands b
INNER JOIN brands_components bcc
ON b.id = bcc.brand_id
INNER JOIN components c ON bcc.component_id = c.id
) brand_query ON brand_names IS NOT NULL
Group by c.name,c.id,t.name,t.id

Answer 3

I got it working with a function:
CREATE FUNCTION brands(int) RETURNS varchar AS $$
SELECT string_agg(b.name, ', ') AS brand_names
FROM brands b
INNER JOIN brands_components
ON b.id = brands_components.brand_id
WHERE brands_components.component_id = $1
$$ LANGUAGE SQL;
SELECT c.name, c.id, t.name AS template_name, t.id AS template_id, brands(c.id), COUNT(p.component_id)
FROM publications p
INNER JOIN components c
ON c.id = p.component_id
INNER JOIN brands_components bc
ON c.id = bc.component_id
AND bc.brand_id IN (1, 2, 3, 4)
INNER JOIN components_templates ct
ON c.id = ct.component_id
INNER JOIN templates t
ON t.id = ct.template_id
GROUP BY 1, 2, 3, 4
Not sure which is preferable, likely it's DineshDB's though.