window functions solve

avysokih/postgresql/window functions/win_func_tasks.sql

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+/*
+Задача 1:
+
+Для каждой категории показать 2 строки:
+- Самый дорогой товар с email'ом
+- Самый дорогой товар без email'а
+*/
+with with_mail as (
+select category, name, price, email_support
+from (
+select category, name, price, email_support,
+row_number() over (partition by category order by price desc) as rn
+from products_extended 
+where email_support is not null
+) t
+where rn = 1
+),
+without_mail as (
+select category, name, price, email_support
+from (
+select category, name, price, email_support,
+row_number() over (partition by category order by price desc) as rn
+from products_extended
+where email_support is null
+) t
+where rn = 1 
+)
+
+select * from with_mail
+union all
+select * from without_mail
+order by category;
+
+/*
+Задача 2:
+
+Для каждого товара показать:
+
+- `name` — название товара
+- `category` — категория товара
+- `price` — цена текущего товара
+- `next_price` — цена следующего товара внутри той же категории, упорядоченного по цене
+- `diff` — разница между ценой текущего товара и ценой следующего товара 
+(`price - next_price`)
+*/
+
+select name, category, price, lead(price) over(partition by category order by price) as next_price, 
+price - lead(price) over(partition by category order by price) as diff
+from products_extended;
+
+/*
+Задача 3:
+
+Показать `category`, `name`, `price`, `cumulative_sales`
+
+Где `cumulative_sales` — это сумма `price` **внутри категории** по мере возрастания цены
+*/
+
+select category, name, price, 
+sum(price) over (partition by category order by price) as cumulative_sales
+from products_extended;
+
+/*
+Задача 4:
+
+Вывести `category`, `name`, `price`, `avg_price`, `is_above_avg`
+
+Где `avg_price` — средняя цена по категории
+
+`is_above_avg = true`, если `price > avg_price`
+
+*/
+
+select  category, name, price, 
+round(avg(price) over (partition by category), 1) as avg_price,
+price > avg(price) over (partition by category) as is_above_avg
+from products_extended;
+
+
+