Java

import java.sql.*;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.ZoneOffset;
//import java.util.Date;
import java.time.format.DateTimeFormatter;
import java.util.Random;

public class Main {

    private Connection connection;

    private Connection getConnection() {
        if (connection == null) {
            try {
                String PASSWORD = "olap";
                String URL = "jdbc:postgresql://localhost:5432/shop";
                String LOGIN = "olap";
                String DRIVER = "org.postgresql.Driver";

                Class.forName(DRIVER);

                connection = DriverManager.getConnection(URL, LOGIN, PASSWORD);
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
            }
        }
        return connection;
    }

    private int getRandomCustomerId() {
        return new Random().nextInt(10) + 1;
    }

    private int getRandomEmployeeId() {
        return new Random().nextInt(10) + 1;
    }

    private int getRandomCommodityId() {
        return new Random().nextInt(31) + 1;
    }

    private int insertInvoice(LocalDate date) {
        try {
            String query = "INSERT INTO invoices(employee_id, customer_id, date) VALUES (?, ?, ?)";
            PreparedStatement statement = getConnection().prepareStatement(query, Statement.RETURN_GENERATED_KEYS);

            statement.setInt(1, getRandomEmployeeId());
            statement.setInt(2, getRandomCustomerId());
            statement.setDate(3, Date.valueOf(date));

            statement.executeUpdate();

            ResultSet rs = statement.getGeneratedKeys();
            if (rs.next()) {
                return rs.getInt(1);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

        return -1;
    }

    private void addItem(int invoiceId) {
        double priceFluctuation = 0.1;
        int maxNumberOfItems = 5;

        try {
            int commodityId = getRandomCommodityId();

            String selectQuery = "SELECT name, price FROM commodity WHERE id=?";
            PreparedStatement selectStatement = getConnection().prepareStatement(selectQuery);

            selectStatement.setInt(1, commodityId);

            ResultSet rs = selectStatement.executeQuery();
            if (rs.next()) {
                System.out.println("\tAdding item " + rs.getString("name"));

                int finalPrice = (int) (rs.getInt("price") * (1 + (Math.random() - 0.5) * 2 * priceFluctuation));

                String insertQuery = "INSERT INTO sold_item(invoice_id, commodity_id, quantity, price) VALUES (?, ?, ?, ?)";
                PreparedStatement insertStatement = getConnection().prepareStatement(insertQuery);

                insertStatement.setInt(1, invoiceId);
                insertStatement.setInt(2, commodityId);
                insertStatement.setInt(3, new Random().nextInt(maxNumberOfItems) + 1);
                insertStatement.setInt(4, finalPrice);

                insertStatement.execute();
            }

        } catch (SQLException e) {
            System.err.println("Duplicate item in invoice " + invoiceId);
        }
    }

    private void generateSales() {
        int salesNumber = 1000;
        int maxNumberOfItems = 5;

        LocalDate startDate = LocalDate.of(2010, 1, 1);
        double incFail = 0.5;

        for (int index = 0; index < salesNumber; index++) {
            if (Math.random() < incFail) {
                startDate = startDate.plusDays(1);
                index--;
            } else {
                System.out.println("New invoice at " + DateTimeFormatter.ofPattern("dd.MM.yyyy").format(startDate));
                int invoiceId = insertInvoice(startDate);
                int itemsCount = new Random().nextInt(maxNumberOfItems) + 1;

                for (int it = 0; it < itemsCount; it++) {
                    addItem(invoiceId);
                }
            }
        }
    }

    private void generateItems() {
        int shelfCount = 20;
        int maxQuantity = 10;
        int operationsCount = 50;


        for (int i = 0; i < operationsCount; i++) {
            try {
                String sql = "INSERT INTO items(commodity_id, shelf_number, quantity) VALUES (?, ?, ?)";
                PreparedStatement statement = getConnection().prepareStatement(sql);

                statement.setInt(1, getRandomCommodityId());
                statement.setInt(2, new Random().nextInt(shelfCount) + 1);
                statement.setInt(3, new Random().nextInt(maxQuantity) + 1);

                statement.execute();
                System.out.println("Added");
            } catch (SQLException e) {
                System.err.println("Duplicate key");
                operationsCount--;
            }
        }

    }

    private void generate() {
        generateItems();
        generateSales();
    }

    public static void main(String[] args) {
        new Main().generate();
    }

}