如何实现检索数据库中的记录不重复注册

我们常常在使用某个网站时候，要成为该网站会员就需要注册一个账号。往往不允许用户名或账号重复的，因此当新用户注册时。如果遇到用户名或账号相同时，就应该提示用户，该账号用户已经存在。那么这个功能怎么来实现的呢？通常有以下几种技巧：

1、用ExecuteScalar()方法。

        private void button1_Click(object sender, EventArgs e)

        {

            if (textBox1.Text.Trim() == "" || textBox2.Text.Trim() == "" || textBox3.Text.Trim() == "")

                MessageBox.Show("请填写完整的信息！", "信息提示");

            else

            {

                sql.sqlConnection.Open();

                string sqluser = "select * from userinfo where username='"+textBox1.Text.Trim()+"'";

                SqlCommand MyCom = new SqlCommand(sqluser,sql.sqlConnection);

                if (null != MyCom.ExecuteScalar())                                                       //检测查询到的结果是否为空，如果不为空则表明该记录存在
                {

                    MessageBox.Show(textBox1.Text.Trim() + "用户名已经存在，请重新注册用户名", "信息提示");

                    textBox1.Text = "";

                    textBox2.Clear();

                    textBox3.Clear();

                    textBox1.Focus();

                }

                else if (textBox2.Text.Trim() == textBox3.Text.Trim())

                {

                    string sqlnewuser = "insert into userinfo(username,userpwd)" + " values('" + textBox1.Text.Trim() + "','" + textBox2.Text.Trim() + "')";

                    MyCom.CommandText = sqlnewuser;

                    MyCom.ExecuteNonQuery();

                    textBox1.Text = "";/*清空文本框*/

                    textBox2.Text = "";

                    textBox3.Text = "";

                    MessageBox.Show("用户添加成功","信息提示");

                    textBox1.Focus();//焦点显示到用户名处

                }

                else

                {

                    MessageBox.Show("两次输入的密码不一致！","信息提示");

                    textBox3.Text = "";

                    textBox3.Focus();//焦点显示到确认密码处

                }

                sql.sqlConnection.Close();

            }

           

        }

2、采用DataSet检测数据表中记录是否存在

      public DataSet GetList(string strWhere)

  {

   StringBuilder strSql=new StringBuilder();

   strSql.Append("select Id,Symbol_Primary,Name_Primary ");

   strSql.Append(" FROM TB_FileClassificationDirectoryone ");
   if(strWhere.Trim()!="")

   {

    strSql.Append(" where "+strWhere);

   }

   return DBHelp.Query(strSql.ToString());

  }

**************************************************************************************************

 public static DataSet Query(string SQLString)//执行语句

        {

            using (SqlConnection connection = new SqlConnection(connectionString))

            {

                DataSet ds = new DataSet();

                try

                {

                    connection.Open();

                    SqlDataAdapter command = new SqlDataAdapter(SQLString, connection);

                    command.Fill(ds, "ds");

                }

                catch (System.Data.SqlClient.SqlException ex)

                {

                    throw new Exception(ex.Message);

                }

                finally

                {

                    connection.Close();

                }

                return ds;

            }

        }

*****************************************************************************************************

  public bool addlist1(string name, string num)

        {

            DataSet ds = S_TB_FileClassificationDirectoryone.GetList("Symbol_Primary='"+ num+ "'");

            if (ds.Tables[0].Rows.Count == 0)   //检测数据表中的记录是否存在，等于0表示为空，不存在该记录。
            {

                EModel.Name_Primary = name;

                EModel.Symbol_Primary = num;

                S_TB_FileClassificationDirectoryone.Add(EModel);//添加记录

                return true;

            }

            else

                return false;

        }

******************************************************************************************************

  public decimal Add(Entity.TB_FileClassificationDirectoryone model)

  {

   StringBuilder strSql=new StringBuilder();

   StringBuilder strSql1=new StringBuilder();

   StringBuilder strSql2=new StringBuilder();

   if (model.Symbol_Primary != null)

   {

    strSql1.Append("Symbol_Primary,");

   ********省略

   }

   strSql.Append("insert into TB_FileClassificationDirectoryone(");

   strSql.Append(strSql1.ToString().Remove(strSql1.Length - 1));

   strSql.Append(")");

   strSql.Append(" values (");

   strSql.Append(strSql2.ToString().Remove(strSql2.Length - 1));

   strSql.Append(")");

   strSql.Append(";select @@IDENTITY");

   object obj = DBHelp.GetSingle(strSql.ToString());///
执行一条计算查询结果语句，返回查询结果（object）。

   if (obj == null)

   {

    return 0;

   }

   else

   {

    return Convert.ToDecimal(obj);

   }

  }

*****************************************************************************

        /// <summary>

        /// 执行一条计算查询结果语句，返回查询结果（object）。

        /// </summary>

        /// <param name="SQLString">计算查询结果语句</param>

        /// <returns>查询结果（object）</returns>

        public static object GetSingle(string SQLString)

        {

            using (SqlConnection connection = new SqlConnection(connectionString))

            {

                using (SqlCommand cmd = new SqlCommand(SQLString, connection))

                {

                    try

                    {

                        connection.Open();

                        object obj = cmd.ExecuteScalar();

                        if ((Object.Equals(obj, null)) || (Object.Equals(obj, System.DBNull.Value)))

                        {

                            return null;

                        }

                        else

                        {

                            return obj;

                        }

                    }

                    catch (System.Data.SqlClient.SqlException e)

                    {

                        connection.Close();

                        throw e;

                    }

                    finally

                    {

                        cmd.Dispose();

                        connection.Close();

                    }

                }

            }

        }