public class DataBase
    {
        public static DataBase DB = new DataBase();
        private NpgsqlConnection conn;
        NpgsqlCommand cmd = new NpgsqlCommand();

        private const string connectionStr = "Host=localhost;Username=postgres;Password=12345678;Database=postgres";

        public void Connect(string connStr = connectionStr)
        {
            conn = new NpgsqlConnection(connStr);
            conn.Open();
            cmd.Connection = conn;
        }
        // language=postgres

        public void InitDB()
        {
            cmd.CommandText = "CREATE TABLE IF NOT EXISTS restaurants(id SERIAL PRIMARY KEY ,name VARCHAR(40))";
            cmd.ExecuteNonQuery();
            cmd.CommandText =
                "CREATE TABLE IF NOT EXISTS workers(id SERIAL PRIMARY KEY,first_name VARCHAR(40),last_name VARCHAR(40),coock_type VARCHAR(30))";
            cmd.ExecuteNonQuery();
            cmd.CommandText =
                "CREATE TABLE IF NOT EXISTS timetable(id SERIAL PRIMARY KEY,restaurant VARCHAR(40),data JSON)";
            cmd.ExecuteNonQuery();

            cmd.CommandText =
                "CREATE TABLE IF NOT EXISTS rus(id SERIAL PRIMARY KEY,restaurant VARCHAR(40),date_week DATE,workers VARCHAR(40)[])";
            cmd.ExecuteNonQuery();

            cmd.CommandText =
                "CREATE TABLE IF NOT EXISTS italy(id SERIAL PRIMARY KEY,restaurant VARCHAR(40),date_week DATE,workers VARCHAR(40)[])";
            cmd.ExecuteNonQuery();

            cmd.CommandText =
                "CREATE TABLE IF NOT EXISTS french(id SERIAL PRIMARY KEY,restaurant VARCHAR(40),date_week DATE,workers VARCHAR(40)[])";
            cmd.ExecuteNonQuery();


        }

        public string[] GetAllRestaurants()
        {
            cmd.Cancel();
            cmd = new NpgsqlCommand("SELECT name FROM restaurants",conn);
            var reader = cmd.ExecuteReader();
            var restaurants = new List<string>();
            while (reader.Read())
            {
                restaurants.Add(reader.GetString(0));
            }

            return restaurants.ToArray();
        }

    }