ДЗ к 6 уроку

HomeWork-Lesson6.sql

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+-- Создать все необходимые внешние ключи. Создать таблицы лайков и постов.
+
+ALTER TABLE profiles
+  ADD CONSTRAINT profiles_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+      ON DELETE CASCADE,
+  ADD CONSTRAINT profiles_photo_id_fk
+    FOREIGN KEY (photo_id) REFERENCES media(id)
+      ON DELETE SET NULL;
+
+ALTER TABLE messages
+  ADD CONSTRAINT messages_from_user_id_fk 
+    FOREIGN KEY (from_user_id) REFERENCES users(id)
+      ON DELETE CASCADE,
+  ADD CONSTRAINT messages_to_user_id_fk 
+    FOREIGN KEY (to_user_id) REFERENCES users(id)
+      ON DELETE CASCADE;
+
+ALTER TABLE friendship
+  ADD CONSTRAINT friendship_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+       ON DELETE CASCADE,
+  ADD CONSTRAINT friendship_friend_id_fk 
+    FOREIGN KEY (friend_id) REFERENCES users(id)
+       ON DELETE CASCADE,
+  ADD CONSTRAINT friendship_status_id_fk 
+    FOREIGN KEY (status_id) REFERENCES friendship_statuses(id)
+       ON DELETE CASCADE;
+
+ALTER TABLE media
+  ADD CONSTRAINT media_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+       ON DELETE CASCADE,
+  ADD CONSTRAINT media_media_type_id_fk 
+    FOREIGN KEY (media_type_id) REFERENCES media_types(id)
+       ON DELETE CASCADE;
+
+ALTER TABLE communities_users
+  ADD CONSTRAINT communities_users_community_id_fk
+    FOREIGN KEY (community_id) REFERENCES communities(id)
+       ON DELETE CASCADE,
+  ADD CONSTRAINT communities_users_community_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+       ON DELETE CASCADE;
+
+ALTER TABLE posts
+  ADD CONSTRAINT posts_media_id_fk
+    FOREIGN KEY (media_id) REFERENCES media(id)
+      ON DELETE CASCADE,      
+  ADD CONSTRAINT posts_community_id_fk
+    FOREIGN KEY (community_id) REFERENCES communities(id)
+      ON DELETE CASCADE,         
+  ADD CONSTRAINT posts_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+      ON DELETE CASCADE;
+
+ALTER TABLE likes
+  ADD CONSTRAINT likes_target_type_id_fk
+    FOREIGN KEY (target_type_id) REFERENCES target_types(id)
+      ON DELETE CASCADE,
+  ADD CONSTRAINT likes_user_id_fk 
+    FOREIGN KEY (user_id) REFERENCES users(id)
+      ON DELETE CASCADE;
+
+
+
+
+
+
+
+
+
+-- Подсчитать общее количество лайков, которые получили 10 самых молодых пользователей.
+
+SELECT * FROM target_types;
+
+SELECT * FROM profiles ORDER BY birthday DESC LIMIT 10;
+
+SELECT 
+  (SELECT COUNT(*) FROM likes WHERE target_id = profiles.user_id AND target_type_id = 2) AS likes_total  
+  FROM profiles 
+  ORDER BY birthday
+ ;
+
+SELECT SUM(likes_total) FROM  
+  (SELECT 
+    (SELECT COUNT(*) FROM likes WHERE target_id = profiles.user_id AND target_type_id = 2) AS likes_total  
+    FROM profiles 
+    ORDER BY birthday 
+    DESC LIMIT 10) AS user_likes
+;  
+
+
+
+
+
+
+
+
+-- Определить кто больше поставил лайков (всего) - мужчины или женщины?
+
+SELECT
+	(SELECT gender FROM profiles WHERE user_id = likes.user_id) AS gender
+    FROM likes; 
+
+SELECT
+	(SELECT gender FROM profiles WHERE user_id = likes.user_id) AS gender,
+	COUNT(*) AS total
+    FROM likes
+    GROUP BY gender
+    ORDER BY total DESC
+    LIMIT 1;
+
+
+
+
+
+-- Найти 10 пользователей, которые проявляют наименьшую активность в использовании социальной сети.
+
+SELECT 
+  CONCAT(first_name, ' ', last_name) AS user, 
+	(SELECT COUNT(*) FROM likes WHERE likes.user_id = users.id) + 
+	(SELECT COUNT(*) FROM media WHERE media.user_id = users.id) + 
+	(SELECT COUNT(*) FROM messages WHERE messages.from_user_id = users.id) AS overall_activity 
+	  FROM users
+	  ORDER BY overall_activity
+	  LIMIT 10;