How to write query in postgresql? (relationship many-to-many) - postgresql

Question

I'm still a newbie.
I created db like this :
DROP DATABASE IF EXISTS image_store_db;
CREATE DATABASE image_store_db;
\c image_store_db;
CREATE TABLE categories_images (
categories_images_id SERIAL PRIMARY KEY,
title VARCHAR NOT NULL,
image_url VARCHAR NOT NULL,
design_url VARCHAR NOT NULL
);
CREATE TABLE images (
images_id SERIAL PRIMARY KEY,
title VARCHAR NOT NULL,
rating REAL NOT NULL,
image_url VARCHAR NOT NULL,
desc_short TEXT NOT NULL,
desc_full TEXT NOT NULL
);
CREATE TABLE ref_categories_images (
categories_images_id integer REFERENCES categories_images (categories_images_id) ON UPDATE CASCADE,
images_id integer REFERENCES images (images_id) ON UPDATE CASCADE ON DELETE CASCADE,
CONSTRAINT ref_categories_images_pkey PRIMARY KEY (images_id, categories_images_id)
);
INSERT INTO categories_images(title, image_url, design_url)
VALUES ('SIMPLE TITLE TEST', '/TEST_URL.PNG', '/TEST_URL.PNG');
INSERT INTO images(title, rating, image_url, desc_short, desc_full)
VALUES ('SIMPLE TITLE TEST', 4.5, '/TEST_URL.PNG', 'TEST_SHORT', 'TEST_FULL');
Pls, help. Teach me, how write a query : insert (for image in category) and select (image from category id) and etc ...
pls ...

Answer 1

My answer assumes that categories_images is the table of categories and images is the table of images. The table and attribute names seem to suggest that the concept of many-to-many join is not all clear to you yet; I would have called the three tables category, image and category_image_map.
For INSERT: if your problem are the serial primary keys, use INSERT ... RETURNING.
You can insert into all three tables in a single statement:
WITH im(im_id) AS (
INSERT INTO categories_images ...
RETURNING categories_images_id
),
cat(cat_id) AS (
INSERT INTO images ...
RETURNING images_id
)
INSERT INTO ref_categories_images (categories_images_id, images_id)
VALUES ((SELECT cat_id FROM cat), (SELECT im_id FROM im));
For the query, you just join the three tables:
SELECT ...
FROM categories_images c
JOIN ref_categories_images r
ON r.categories_images_id = c.categories_images_id
JOIN images i
ON r.images_id = i.images_id
and add a WHERE clause for your condition, for example
WHERE c.categories_images_id = 42
or
WHERE i.image_title = 'Mona Lisa'