如何实现多个条件进行搜索？

private void btnSearch_Click(object sender, System.EventArgs e)
        {
//首先需要建立一个StringBuilder对象
StringBuilder sql = new StringBuilder();

            // Limit maximum resultset size
            sql.Append(@"SELECT TOP ");
            sql.Append(ConfigurationSettings.AppSettings["searchLimit"]);
            sql.Append(@"
          [User].UserID, [User].FirstName, [User].LastName, 
          Place.PlaceID, Place.Name AS PlaceName, 
          PlaceType.Name AS PlaceType, PlaceType.TypeID,
          TimeLapse.Name AS LapseName, TimeLapse.YearIn, 
          TimeLapse.MonthIn, TimeLapse.YearOut, TimeLapse.MonthOut 
        FROM [User] 
        LEFT OUTER JOIN TimeLapse ON 
          TimeLapse.UserID = [User].UserID
        LEFT OUTER JOIN Place ON
          Place.PlaceID = TimeLapse.PlaceID 
        LEFT OUTER JOIN PlaceType ON
          Place.TypeID = PlaceType.TypeID 
        ");

            // 生成WHERE语句Build the WHERE clause now
            StringBuilder qry = new StringBuilder();
            if (txtFirstName.Text != String.Empty)
            {
                qry.Append("[User].FirstName LIKE '%");
                qry.Append(txtFirstName.Text).Append("%' AND ");
            }
            if (txtLastName.Text != String.Empty)
            {
                qry.Append("[User].LastName LIKE '%");
                qry.Append(txtLastName.Text).Append("%' AND ");
            }
            if (cbPlace.SelectedItem.Value != "0")
            {
                qry.Append("[Place].PlaceID = '");
                qry.Append(cbPlace.SelectedItem.Value).Append("' AND ");
            }
            if (cbType.SelectedItem.Value != "0")
            {
                qry.Append("[PlaceType].TypeID = '");
                qry.Append(cbType.SelectedItem.Value).Append("' AND ");
            }
            if (txtYearIn.Text != String.Empty)
            {
                qry.Append("TimeLapse.YearIn = ");
                qry.Append(txtYearIn.Text).Append(" AND ");
            }
            if (txtYearOut.Text != String.Empty)
            {
                qry.Append("TimeLapse.YearOut = ");
                qry.Append(txtYearOut.Text).Append(" AND ");
            }
//根据发表日期的ddlPostTime的下拉列表中所选定的值与当前时间计算时间范围
            //如果为所有日期（ddlPostTime.SelectedValue="0"）则将日期调为２０００年，否则按当前时间减selectedvalue的天数成为查询的日期值
        //    DateTime qryDateTime=new DateTime();
        //    if(ddlPostTime.SelectedValue=="0")
        //    {
        //        qryDateTime=new System.DateTime(1999, 12, 31, 23, 59, 0);

        //    }
        //    else
        //    {
        //        Double days=Convert.ToDouble(ddlPostTime.SelectedValue);
        //        qryDateTime=DateTime.Today.AddDays(days);
        //    }
        //    qry.Append("posttime >=#");      //使用时间需加#
        //    qry.Append(qryDateTime).Append("# AND ");
           //以下语句可构成多条件中一个或多个条件进行搜索
            string filter = qry.ToString();
            if (filter.Length != 0) 
            {
                sql.Append(" WHERE ");

                // Add the filter without the trailing AND
                sql.Append(filter.Remove(filter.Length - 4, 4));
            }

            SqlDataAdapter ad = new SqlDataAdapter(sql.ToString(), cnFriends);
            dsResults = new DataSet();
            ad.Fill(dsResults, "User");

            // Adjust label for results
            if (dsResults.Tables["User"].Rows.Count < 
                Convert.ToInt32(ConfigurationSettings.AppSettings["searchLimit"]))
            {
                lblLimit.Text = "Found " + 
                    dsResults.Tables["User"].Rows.Count.ToString() + 
                    " users matching your criteria on initial search.";
            }
            else
            {
                lblLimit.Text = "You're working with the first " + 
                    ConfigurationSettings.AppSettings["searchLimit"] +
                    @" results.  If you're looking for someone who's not in this list,
         please search again with a more precise search criterion.";
            }

            // Place results in session state
            Session["search"] = dsResults;

            SetResultsState(true);
        }