sql话语多条件查询

sql语句多条件查询?
,如图所示，不知道图片能否传上去，我想实现的功能是，当文本框为空时，查询所有满足是否在线和是否允许发言的用户，当输输入用户名后按这个三个条件查询，用户名、是否在线、是否允许发言，怎样写sql语句啊，我这是一部分，有错误，
 string txtUserName = this.TextBox1.Text.ToString().Trim();
  string sqlText = "select name,isonline,isallowmessage from userinformation where 1=1";
   
   
  if (txtUserName == "")
  {
  sqlText += "";
  }
  else if (this.DropDownList1.SelectedValue == "在线")
  {
  sqlText += "and UserName=@userName and IsOnline=true";
  }
  else if(this.DropDownList2.SelectedValue=="可以"){
  sqlText += "and IsAllowMessage=true";
  }
   

  OleDbConnection con = DB.createCon();
  con.Open();

  DataTable table = new DataTable();
  OleDbDataAdapter dataAdapter = new OleDbDataAdapter(sqlText,con );

  OleDbParameter para1 = new OleDbParameter("@userName", OleDbType.WChar, 50);
  para1.Value = txtUserName;
  dataAdapter.SelectCommand.Parameters.Add(para1);
  dataAdapter.Fill(table);

  this.GridView1.DataSource = table.DefaultView;
  this.GridView1.DataBind();
 

[解决办法]
string sqlText = "select name,isonline,isallowmessage from userinformation where 1=1"; 
if (txtUserName == "") 
{ 
sqlText += ""; 
} 
sqlText += this.DropDownList1.SelectedValue.Equals("在线")?"and UserName=@userName and IsOnline=true":"and UserName=@userName and IsOnline=false"; 
sqlText += "and UserName=@userName and IsOnline=false"; 
sqlText += this.DropDownList2.SelectedValue.Equals("可以")?"IsAllowMessage=true":"and IsAllowMessage=false";