How to find all the products with specific multi attribute values - sql

Question

I am using postgresql.
I have a table called custom_field_answers. The data looks like this
Id | product_id | value | number_value |
4 | 2 | | 117 |
3 | 1 | | 107 |
2 | 1 | bangle | |
1 | 2 | necklace | |
I want to find all the products which has text_value as 'bangle' and number_value less than 50.
Here was my first attempt.
SELECT "products".* FROM "products" INNER JOIN "custom_field_answers"
ON "custom_field_answers"."product_id" = "products"."id"
WHERE ("custom_field_answers"."value" ILIKE 'bangle')
Here is my second attempt.
SELECT "products".* FROM "products" INNER JOIN "custom_field_answers"
ON "custom_field_answers"."product_id" = "products"."id"
where ("custom_field_answers"."number_value" < 50)
Here is my final attempt.
SELECT "products".* FROM "products" INNER JOIN "custom_field_answers"
ON "custom_field_answers"."product_id" = "products"."id"
WHERE ("custom_field_answers"."value" ILIKE 'bangle')
AND ("custom_field_answers"."number_value" < 50)
but this does not select any product record.

Answer 1

A WHERE clause can only look at columns from one row at a time.
So if you need a condition that applies to two different rows from a table, you need to join to that table twice, so you can get columns from both rows.
SELECT p.*
FROM "products" AS p
INNER JOIN "custom_field_answers" AS a1 ON p."id" = a1."product_id"
INNER JOIN "custom_field_answers" AS a2 ON p."id" = a1."product_id"
WHERE a1."value" = 'bangle' AND a2."number_value" < 50

Answer 2

It produces no records because there is no custom_field_answers record that meets both criteria. What you want is a list of product_ids that have the necessary records in the table. Just in case no one gets to writing the SQL for you, and until I have a chance to work it out myself, I thought I would at least explain to you why your query is not working.

Answer 3

This should work:
SELECT p.* FROM products LEFT JOIN custom_field_answers c
ON (c.product_id = p.id AND c.value LIKE '%bangle%' AND c.number_value
Hope it helps

Answer 4

Your bangle-related number_value fields are null, so you won't be able to do a straight comparison in those cases. Instead, convert your nulls to 0s first.
SELECT "products".* FROM "products" INNER JOIN "custom_field_answers"
ON "custom_field_answers"."product_id" = "products"."id"
WHERE ("custom_field_answers"."value" LIKE '%bangle%')
AND (coalesce("custom_field_answers"."number_value", 0) < 50)

Answer 5

Didn't actually test it, but this general idea should work:
SELECT *
FROM products
WHERE
EXISTS (
SELECT *
FROM custom_field_answers
WHERE
custom_field_answers.product_id = products.id
AND value = 'bangle'
)
AND EXISTS (
SELECT *
FROM custom_field_answers
WHERE
custom_field_answers.product_id = products.id
AND number_value < 5
)
In plain English: Get all products such that...
there is a related row in custom_field_answers where value = 'bangle'
and there is (possibly different) related row in custom_field_answers where number_value < 5.