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