fix columns in tables

vberetar/postgresql/ddl.sql

@@ -1,8 +1,8 @@
 CREATE TABLE users (
-	id INT PRIMARY KEY NOT NULL,
+	id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
 	name VARCHAR(100) NOT NULL,
-	age SMALLINT,
+	age SMALLINT CHECK (age > 0),
-	email VARCHAR(100),
+	email VARCHAR(100) NOT NULL UNIQUE,
 	country VARCHAR(50),
 	is_active BOOLEAN DEFAULT true,
 	created_at TIMESTAMPTZ NOT NULL
@@ -13,27 +13,26 @@ CREATE TABLE users (
 -- id - если не прописать not null в primary key, то not null ставится автоматически?
 
 CREATE TABLE products (
-	id INT PRIMARY KEY,
+	id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
 	name VARCHAR(100) NOT NULL,
 	category VARCHAR(50),
 	price DECIMAL(10, 2) NOT NULL,
-	in_stock BOOLEAN
+	in_stock BOOLEAN DEFAULT true,
 );
 
 CREATE TABLE orders(
-	id INT PRIMARY KEY,
+	id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
 	user_id INT,
 	order_date TIMESTAMPTZ NOT NULL,
 	status VARCHAR(20),
-	FOREING KEY (user_id) REFERENCES users(id)
+	FOREIGN KEY (user_id) REFERENCES users(id)
 );
 -- status - лучше прописать словами статус, так понятней будет. Я бы сюда поставил ENUM
 
 CREATE TABLE order_items(
-	order_id INT,
+	order_id INT NOT NULL,
-	product_id INT,
+	product_id INT NOT NULL,
 	quantity INT NOT NULL,
 	price_at_purchase DECIMAL(10, 2) NOT NULL,
-	FOREING KEY (order_id) REFERENCES orders(id),
+	PRIMARY KEY (order_id, product_id)
-	FOREING KEY (product_id) REFERENCES products(id)
 );