Таблицы и их связи для месенджера на сервере

SELECT Users.name, UserChat.chat_id FROM Users, UserChat WHERE Users.user_id = ('User1');

SELECT usres.name, user_chat.user_id, user_chat.chat_id FROM user_chat JOIN users ON users.id=user_chat.user_id WHERE users.id=N;

SELECT user_id, chat_id FROM UserChat WHERE chat_id IN (SELECT chat_id FROM UserChat WHERE user_id = 1);

SELECT
    USER.name AS username, chat_id
FROM UserChat
    LEFT JOIN Users AS USER ON USER.user_id = UserChat.user_id
WHERE chat_id IN
      (SELECT chat_id FROM UserChat WHERE user_id = 1);

SELECT * FROM tab ORDER BY name DESC;

SELECT name, "numberChat"
FROM "UserChat"
LEFT JOIN (SELECT id AS user_id, name FROM "Users") u USING (user_id)
LEFT JOIN (SELECT id AS chat_id, chat AS "numberChat" FROM "Chat") c USING (chat_id)

AS chat_id, chat

SELECT id AS chat_id, chat AS "numberChat"

SELECT master_name, "numberChat", user_name FROM
-- Выбираем пользователя 'UserTest1' - хозяина чатов
(SELECT id AS master_id, name AS master_name FROM "Users" WHERE name = 'UserTest1') AS master
-- Выясняем, какие чаты у него есть
INNER JOIN (SELECT id AS user_id, user_name FROM "UserChat") AS chat ON (user_id = master_id)
-- Находим пользователей, участвующих в этих чатах
LEFT JOIN (SELECT id AS user_id, name AS user_name FROM "Users") AS USER USING (user_id)

WHERE name = 'UserTest1'

SELECT master_name, "numberChat", user_name FROM
-- Выбираем пользователя 'UserTest1' - хозяина чатов
(SELECT id AS master_id, name AS master_name FROM "Users" WHERE name = 'UserTest1') AS master
-- Выясняем, какие чаты у него есть
INNER JOIN (SELECT user_id AS master_id, chat_id AS master_chat_id FROM "UserChat") AS master_chat USING (master_id)
-- Проходимся по всем пользователям, которые могут участвовать в каждом из этих чатов
CROSS JOIN (SELECT id AS user_id, name AS user_name FROM "Users") AS userr
-- Отмечаем тех пользователей, которые действительно участвуют
LEFT JOIN (SELECT user_id, chat_id AS user_chat_id FROM "UserChat") AS chat_user USING (user_id)