Add first ddl

SQL/ddl.sql

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+-- Таблица пользователей
+create table akalyanov.users (
+    id int primary key,
+    name varchar(100) not null,
+    age smallint,
+    email varchar(100),
+    country varchar(2),
+    is_active boolean,
+    created_at timestamp not null
+);
+-- Таблица товаров
+create table akalyanov.products (
+    id int primary key,
+    name varchar(100) not null,
+    category varchar(50),
+    price decimal(10, 2) not null,
+    in_stock boolean
+);
+-- Таблица заказов
+create table akalyanov.orders (
+    id int primary key,
+    user_id int,
+    order_date date not null,
+    status varchar(20),
+    foreign key (user_id) references akalyanov.users(id)
+);
+-- Таблица товаров в заказе
+create table akalyanov.order_items (
+    order_id int,
+    product_id int,
+    quantity smallint not null,
+    price_at_purchase decimal(10, 2) not null,
+    primary key (order_id, product_id)
+);