Trouble using MySQL

where I made a mistake?

Code:

[Oxide] 19:26 [Error] SMSPlugin plugin failed to compile!
[Oxide] 19:26 [Error] SMSPlugin.cs(19,30): error CS0234: The type or namespace name `MySql' does not exist in the namesp
ace `Oxide.Ext'. Are you missing an assembly reference?

Code:

using System.IO;
using System.Collections.Generic;
using Oxide;
using Oxide.Core;
using System.Text;namespace Oxide.Plugins
{
    [Info("test", "Unknown", 0.1)]
    [Description("test")]    public class test: HurtworldPlugin
    {        private readonly Ext.MySql.Libraries.MySql _mySql = Interface.GetMod().GetLibrary<Ext.MySql.Libraries.MySql>();
        private Core.Database.Connection _mySqlConnection;
        private const string InsertData = "INSERT INTO shop (`nick`, `ranga`, `data`) VALUES (@0, @1, @2);";
        private const string SelectData = "SELECT `nick`, `ranga`, `data` FROM shop;";        private void Init()
        {            Puts("Logowanie do bazy danych");
            _mySqlConnection = _mySql.OpenDb("localhost", 3306, "hurt", "root", "***********", this);            var sql = Core.Database.Sql.Builder.Append(@"DROP TABLE IF EXISTS shop;
                                 CREATE TABLE IF NOT EXISTS shop (
                                 id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
                                 data VARCHAR(100),
                                 data_other INT
                               );");            var r = new System.Random();            for (var i = 0; i < 10; i++)
            {
                var rInt = r.Next();
                sql.Append(InsertData, "test" + rInt, rInt);
            }
            _mySql.Insert(sql, _mySqlConnection);            sql = Core.Database.Sql.Builder.Append(SelectData);
            _mySql.Query(sql, _mySqlConnection, list =>
            {
                if (list == null) return;
                var sb = new StringBuilder();
                sb.AppendLine();
                sb.AppendLine(" id\tdata\t\tdata_other");
                foreach (var entry in list)
                {
                    sb.AppendFormat(" {0}\t{1}\t{2}", entry["id"], entry["data"], entry["data_other"]);
                    sb.AppendLine();
                }
                Puts(sb.ToString());
            });
        }