PostgreSQL : how to query from 3 tables (with 1 junction table)? - sql

Question

table 'product'
------------------------------------------
| id | product_name | product_description|
------------------------------------------
| 1. | abc | this is abc's desc |
------------------------------------------
Junction table 'ingredient_product'
------------------------------------------
| id | product_id | ingredient_id |
------------------------------------------
| 1 | 1 | 1 |
| 1 | 1 | 2 |
| 1 | 1 | 3 |
| 1 | 1 | 4 |
| 1 | 1 | 5 |
| 1 | 1 | 6 |
------------------------------------------
table 'ingredient'
------------------------
| id | ingredient_name |
------------------------
| 1 | apple |
| 2 | chicken |
| 3 | beef |
| 4 | beet |
| 5 | oat |
| 6 | pea fibre |
------------------------
I have 3 tables and tried to query like below
SELECT
product.name AS product_name,
product.description AS product_description,
product.created_at AS product_created,
ingredient.name AS ingredient_name
FROM
product
JOIN
ingredient_product
ON
ingredient_product.product_id = product.id
JOIN
ingredient
ON
ingredient.id = ingredient_product.ingredient_id
WHERE
ingredient_product.product_id = 1;
and I get the result like below
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "banana"}
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "strawberry"}
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "canola oil"}
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "pilchard"}
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "ground beef"}
{product_name: "fromm gold", product_description: "For puppies and pregnant or nursing mothers. Taste… aid digestion and salmon oil for a healthy coat.", ingredient_name: "cranberry"}
I get the all different ingredients but wanted to show only once for the duplicates.
is there better way to query this type?
Thank you in advance!

Answer 1

You seem to want aggregation, something like this:
SELECT p.name AS product_name, p.description AS product_description,
p.created_at AS product_created,
ARRAY_AGG(i.name) AS ingredient_names
FROM product p JOIN
ingredient_product ip
ON ip.product_id = p.id JOIN
ingredient i
ON i.id = ip.ingredient_id
WHERE p.id = 1
GROUP BY p.id;
Notes:
Table aliases make the query easier to write and read.
The WHERE clause filters on the primary key of product rather than on the equivalent column in ingredient_product. I think that primary keys may help the optimizer.
This adds a GROUP BY, because you want one row per product. This is aggregating by the primary key, so the SELECT can contain other columns.
The array_agg() brings the ingredients together as an array.

Answer 2

Do you want to list the product details only once, together with a list of ingredients?
Have a look at aggregate functions e.g. there: https://www.postgresql.org/docs/9.5/functions-aggregate.html .
I think function string_agg will help you.