Add dml.sql and fix ddl.sql

SQL/ddl.sql

@@ -8,19 +8,19 @@ create table akalyanov.users (
     is_active boolean,
     created_at timestamp not null
 );
--- Таблица товаров
+-- Таблица продуктов
 create table akalyanov.products (
     id int primary key,
     name varchar(100) not null,
     category varchar(50),
-    price decimal(10, 2) not null,
+    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,
+    order_date timestamptz not null,
     status varchar(20),
     foreign key (user_id) references akalyanov.users(id)
 );
@@ -29,6 +29,8 @@ 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)
+    price_at_purchase decimal(10,2) not null,
+    primary key (order_id, product_id),
+    foreign key (order_id) references akalyanov.orders(id),
+    foreign key (product_id) references akalyanov.products(id)
 );