fixed mistakes

dmedvedev/10.1.Transactions.sql

@@ -1,14 +1,19 @@
 --Задание 1. Оформление заказа
 BEGIN;
 
-INSERT INTO orders (user_id, order_date, status)
-VALUES (10, '2025-01-15', 'paid');
+    INSERT INTO orders
+        (user_id, order_date, status)
+    VALUES
+        (10, '2025-01-15', 'paid');
 
-INSERT INTO order_items (product_id, quantity, price_at_purchase)
-VALUES(6, 1, 4800), (7, 1, 5200);
+    INSERT INTO order_items
+        (order_id, product_id, quantity, price_at_purchase)
+    VALUES
+        (LASTVAL(), 6, 1, 4800),
+        (LASTVAL(), 7, 1, 5200);
 
     UPDATE products
-SET in_stock = TRUE
+    SET in_stock = FALSE
     WHERE id IN (6, 7);
 
     COMMIT;

dmedvedev/10.2.Blocks.sql

@@ -20,7 +20,7 @@
 --просто прочитает данные. Боб лишь заблокирован для изменений.
 
 --что будет с UPDATE users SET country = 'FR' WHERE id = 2;.
---В таком случае БД увидит конфликт и одну из транзакций откатит с ошибкой , а вторую пропустит дальше.
+--В таком случае БД зависнет (blocked) и будет ждать пока первая транзакция не исполнится.
 
 --Задание 4. Дедлок на двух пользователях
 --Объясни своими словами, почему возникает дедлок и что сделает база с одной из транзакций.

dmedvedev/10.3.Optimization of select.sql

@@ -50,8 +50,8 @@ WHERE EXTRACT(YEAR FROM created_at) = 2026
 SELECT id, name, age
 FROM users 
 WHERE 
-	created_at >= '2024-01-01' AND created_at < '2025-01-01'
-	AND country = 'RU'
+	created_at >= '2026-01-01' AND created_at < '2027-01-01'
+	AND country = 'россия'
 	AND age > 25;
 
 Исправлено: выдача колонок "все" заменено на "определенные", логика в where с числами переработана, соответствие определенному году переработано и убрал функцию LOWER.

dmedvedev/10.4.Explain.sql

@@ -1,12 +1,18 @@
 --Задача 1 — Читаем план
+
+EXPLAIN
+SELECT name, email 
+FROM users 
+WHERE id = 1;
+
 Какой тип сканирования использует база?
 --SeqScan - сканирование всей таблицы
 
 Что означает строка `Index Cond: (id = 1)`?
---  У меня нет такой строки, есть "Filter: (id = 1)". Видимо это фильтрация по определенному значению, но в моем случае это должен быть индекс по столбцу id, но у меня индексы не проставлены, хорошо, что я хотя бы сейчас это уже понимаю.
+--  Строка означает фильтрацию столбца по индексу, который есть в столбце id.
 
 Почему база использует индекс, а не читает всю таблицу?
---Потому что так быстрее и менее затратно.
+--В моем случае БД читает всю таблицу, но обычно если индексы расставлены верно и таблица больших размеров, то БД лучше использовать индексы для повышенной производительности.
 ----------
 
 --Задача 2 — Seq Scan vs Index Scan

dmedvedev/11.4 Pet project Cohort_analysis.sql

@@ -6,7 +6,8 @@ SELECT
 	COUNT(DISTINCT pu.id) AS users_in_cohort,
 	COUNT(DISTINCT pe.user_id) AS enrolled_users,
 	COUNT(DISTINCT plv.user_id) AS active_users,
-	ROUND(COUNT(DISTINCT plv.user_id) * 100.0 / COUNT(DISTINCT pu.id), 2) || '%' AS retention_rate,
+	--ROUND(COUNT(DISTINCT plv.user_id) * 100.0 / COUNT(DISTINCT pu.id), 2) || '%' AS retention_rate,
+	ROUND(COUNT(pe.id) * 100.0 / COUNT(DISTINCT pu.id), 2) || '%' AS retention_rate,
 	ROUND(AVG((SELECT COUNT(pe.id)
 	FROM pet_enrollments pe)) / COUNT(DISTINCT pu.id), 2) AS avg_courses_per_user,
 	(SELECT

dmedvedev/11.5 Pet project Lesson_popularity_ranking.sql

@@ -5,8 +5,8 @@ SELECT
 	pl.title AS lesson_title,
 	pc.id AS course_id,
 	pc.title AS course_title,
-	total_views_lesson.total_views AS total_views,
-	table_unique_viewers.unique_v AS unique_viewers,
+	COALESCE(total_views_lesson.total_views, 0) AS total_views,
+	COALESCE(table_unique_viewers.unique_v, 0) AS unique_viewers,
 	ROUND(AVG(views_one_user_per_one_lesson.views_per_viewer), 2) AS avg_views_per_viewer, --также это можно было посчитать total_views/unique_viewers
 	DENSE_RANK() OVER (
 		PARTITION BY pc.id

dmedvedev/11.6 Pet project Conversion_funnel.sql

@@ -1,47 +1,49 @@
-CREATE MATERIALIZED VIEW conversion_funnel AS
+CREATE MATERIALIZED
+VIEW conversion_funnel AS
 
 -- CTE какое кол-во человек записалось на курс 
 WITH
 
-enrolled_users AS (
+	enrolled_users
+	AS
+	(
 		SELECT
 			course_id,
-		COUNT(*) AS enrolled
+			user_id,
+			COUNT(*) OVER (PARTITION BY course_id) AS enrolled_count
 		FROM pet_enrollments pe
-	GROUP BY course_id
 	),
 
 	-- СТЕ на начавших смотреть курс
-started_users AS (
+	started_users
+	AS
+	(
 		SELECT
-		pc.id AS course_id,
+			eu.course_id,
 			COUNT(DISTINCT plv.user_id) AS started
-	FROM pet_courses pc
+		FROM enrolled_users eu -- Базой берем записанных пользователей
 			LEFT JOIN pet_lessons pl
-		ON pc.id = pl.course_id
+			ON eu.course_id = pl.course_id
 			LEFT JOIN pet_lesson_views plv
-		ON plv.lesson_id = pl.id
-	GROUP BY pc.id
+			ON plv.lesson_id = pl.id AND plv.user_id = eu.user_id
+		-- Проверяем, что смотрел именно записанный юзер
+		GROUP BY eu.course_id
 	),
 
 
 	--СТЕ на кол-во просмотров курса и прогресс по курсу
-user_progress AS (
+	user_progress
+	AS
+	(
 		SELECT
-		pc.id AS course_id,
-		plv.user_id,
-		COUNT(DISTINCT lesson_id) AS count_views,
-		count_lesson_in_course.count_lesson_in_course AS count_lesson_in_course,
-		COUNT(DISTINCT lesson_id) * 1.0 / count_lesson_in_course AS progress
-	
-	FROM pet_courses pc
-
+			eu.course_id,
+			eu.user_id,
+			COUNT(DISTINCT plv.lesson_id) * 1.0 / count_lesson_in_course.count_lesson_in_course AS progress
+		FROM enrolled_users eu -- Базой берем записанных пользователей
 			LEFT JOIN pet_lessons pl
-		ON pl.course_id = pc.id
-
+			ON pl.course_id = eu.course_id
 			LEFT JOIN pet_lesson_views plv
-		ON plv.lesson_id = pl.id
-
+			ON plv.lesson_id = pl.id AND plv.user_id = eu.user_id -- Просмотры могут быть NULL, если записался, но не смотрел
 			LEFT JOIN (
 		SELECT
 				pl.course_id,
@@ -49,20 +51,20 @@ user_progress AS (
 			FROM pet_lessons pl
 			GROUP BY pl.course_id
 	) AS count_lesson_in_course
-		ON count_lesson_in_course.course_id = pc.id
-
-
-	GROUP BY pc.id, plv.user_id, count_lesson_in_course
-	
+			ON count_lesson_in_course.course_id = eu.course_id
+		GROUP BY eu.course_id, eu.user_id, count_lesson_in_course.count_lesson_in_course
 	),
 
-main_table AS (
+	main_table
+	AS
+	(
 											SELECT
 				pc.id AS course_id,
 				pc.title AS course_title,
 				'registered' AS stage_name,
 				0 AS stage_order,
-		(SELECT COUNT(*) FROM pet_users pu) AS users_count,
+				(SELECT COUNT(*)
+				FROM pet_users pu) AS users_count,
 				ROUND(100, 2) || '%' AS conversion_from_previous,
 				ROUND(100, 2) || '%' AS conversion_from_start
 
@@ -75,13 +77,14 @@ main_table AS (
 				pc.title AS course_title,
 				'enrolled' AS stage_name,
 				1 AS stage_order,
-		enrolled_users.enrolled,
-		ROUND(enrolled_users.enrolled * 100.0 / (SELECT COUNT(*) FROM pet_users pu), 2) || '%' AS conversion_from_previous,
-		ROUND(enrolled_users.enrolled * 100.0 / (SELECT COUNT(*) FROM pet_users pu), 2) || '%' AS conversion_from_start
-
+				COALESCE(eu.enrolled_count, 0),
+				ROUND(COALESCE(eu.enrolled_count, 0) * 100.0 / NULLIF((SELECT COUNT(*) FROM pet_users), 0), 2) || '%' AS conversion_from_previous,
+				ROUND(COALESCE(eu.enrolled_count, 0) * 100.0 / NULLIF((SELECT COUNT(*) FROM pet_users), 0), 2) || '%' AS conversion_from_start
 			FROM pet_courses pc
-	LEFT JOIN enrolled_users
-		ON enrolled_users.course_id = pc.id
+				LEFT JOIN (SELECT course_id, MAX(enrolled_count) AS enrolled_count
+				FROM enrolled_users
+				GROUP BY course_id) eu
+				ON eu.course_id = pc.id
 
 
 		UNION ALL
@@ -91,15 +94,16 @@ main_table AS (
 				pc.title AS course_title,
 				'started' AS stage_name,
 				2 AS stage_order,
-		started_users.started,
-		ROUND(started_users.started * 100.0 / enrolled_users.enrolled, 2) || '%' AS conversion_from_previous,
-		ROUND(started_users.started * 100.0 / (SELECT COUNT(*) FROM pet_users pu), 2) || '%' AS conversion_from_start
-
+				COALESCE(su.started, 0),
+				ROUND(COALESCE(su.started, 0) * 100.0 / NULLIF(eu.enrolled_count, 0), 2) || '%' AS conversion_from_previous,
+				ROUND(COALESCE(su.started, 0) * 100.0 / NULLIF((SELECT COUNT(*) FROM pet_users), 0), 2) || '%' AS conversion_from_start
 			FROM pet_courses pc
-	LEFT JOIN started_users
-		ON started_users.course_id = pc.id	
-	LEFT JOIN enrolled_users
-		ON enrolled_users.course_id = pc.id
+				LEFT JOIN (SELECT course_id, MAX(enrolled_count) AS enrolled_count
+				FROM enrolled_users
+				GROUP BY course_id) eu
+				ON eu.course_id = pc.id
+				LEFT JOIN started_users su
+				ON su.course_id = pc.id
 
 		UNION ALL
 
@@ -108,17 +112,18 @@ main_table AS (
 				pc.title AS course_title,
 				'mid_progress' AS stage_name,
 				3 AS stage_order,
-		(	SELECT COUNT(*) FROM user_progress
-			WHERE user_progress.course_id = pc.id AND user_progress.progress >= 0.5
-		),
-		ROUND((	SELECT COUNT(*) FROM user_progress
-				WHERE user_progress.course_id = pc.id AND user_progress.progress >= 0.5) * 100.0 / started_users.started, 2) || '%' AS conversion_from_previous,
-		ROUND((	SELECT COUNT(*) FROM user_progress
-				WHERE user_progress.course_id = pc.id AND user_progress.progress >= 0.5) * 100.0 / (SELECT COUNT(*) FROM pet_users pu), 2) || '%' AS conversion_from_start
-		
+				(SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 0.5),
+				ROUND((SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 0.5) * 100.0 / NULLIF(su.started, 0), 2) || '%' AS conversion_from_previous,
+				ROUND((SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 0.5) * 100.0 / NULLIF((SELECT COUNT(*) FROM pet_users), 0), 2) || '%' AS conversion_from_start
 			FROM pet_courses pc
-	LEFT JOIN started_users
-		ON started_users.course_id = pc.id	
+				LEFT JOIN started_users su
+				ON su.course_id = pc.id
 
 		UNION ALL
 
@@ -127,18 +132,16 @@ main_table AS (
 				pc.title AS course_title,
 				'completed' AS stage_name,
 				4 AS stage_order,
-		(	SELECT COUNT(*) FROM user_progress
-			WHERE user_progress.course_id = pc.id AND user_progress.progress >= 1),
-		ROUND((	SELECT COUNT(*) FROM user_progress 
-				WHERE user_progress.course_id = pc.id AND user_progress.progress >= 1) * 100.0 / 
-			(	SELECT COUNT(*) FROM user_progress
-				WHERE user_progress.course_id = pc.id AND user_progress.progress >= 0.5), 2) || '%' AS conversion_from_previous,
-		ROUND((	SELECT COUNT(*) FROM user_progress 
-				WHERE user_progress.course_id = pc.id AND user_progress.progress >= 1) * 100.0 / 
-				(SELECT COUNT(*) FROM pet_users pu), 2) || '%' AS conversion_from_start
+				(SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 1),
+				ROUND((SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 1) * 100.0 / NULLIF((SELECT COUNT(*) FROM user_progress up WHERE up.course_id = pc.id AND up.progress >= 0.5), 0), 2) || '%' AS conversion_from_previous,
+				ROUND((SELECT COUNT(*)
+				FROM user_progress up
+				WHERE up.course_id = pc.id AND up.progress >= 1) * 100.0 / NULLIF((SELECT COUNT(*) FROM pet_users), 0), 2) || '%' AS conversion_from_start
 			FROM pet_courses pc
-	
-	ORDER BY course_id, stage_order
 	)
 -- main select
 SELECT
@@ -149,4 +152,5 @@ SELECT
 	conversion_from_previous,
 	conversion_from_start
 
-FROM main_table;
+FROM main_table
+ORDER BY course_id, stage_order;

dmedvedev/11.7 Pet Project Additional tasks.sql

@@ -1,14 +1,24 @@
 -- 1. Задание 1: Анализ drop-off точек
 -- Используя conversion_funnel, найди курсы с самым большим процентом отвала между этапами enrolled → started (WHERE stage_name IN (...))
+WITH
+	drop_off
+	AS
+	(
 		SELECT
 			course_id,
 			stage_name,
-	conversion_from_previous,
-	conversion_from_start
-
+			users_count,
+			LAG(users_count) OVER (PARTITION BY course_id) AS prev
 
 		FROM conversion_funnel
-WHERE stage_name IN ('started');
+	)
+
+SELECT
+	course_id,
+	ROUND(1 - users_count * 1.0 / NULLIF(prev, 0), 2) * 100 || '%' AS percent_of_drop
+FROM drop_off
+WHERE stage_name = 'started'
+ORDER BY percent_of_drop DESC
 
 -- ОТВЕТ: Самые большие проценты отвала между enrolled и started замечены на курсе 1 и 3 (62 и 67% соответственно).