asp.net数据库操作技巧例子，改变sql语句

public DataTable dt(string aid,string sid,string lx,string dtt,string key,string big)
    {//根据传来的地区分类,大分类,小分类,读取info数据库的内容
        //下面的镶套判断本来没必要的,呵呵,只是我用了好久才弄完,纪念下
        
         String SQL ="select * from [info] where sh=0" ;
        if (aid != null)
        {//如果地区ID存在
             SQL += " and [aid]=" + aid + "";
            if (sid != null)
            {//小类ID 也 存在
                 SQL += " and [sid]=" + sid + "";
             }
            if (lx != null)
            {//如果类型存在
                     SQL += " and [lx]='" + lx + "'";
             }
         }
        if (sid != null)
        {//如果小类ID存在
            if(aid == null)
             SQL += " and [sid]=" + sid + "";
            if (lx != null)
            {//如果类型存在
                if (aid == null)
                     SQL += " and [lx]='" + lx + "'";
             }
         }
        if (lx != null)
        {//如果类型存在
            if (aid == null&&sid==null)
                 SQL += " and [lx]='" + lx + "'";
         }
        if (dtt != null)
        {
             SQL += " and [dtimme]>= #"+dtt+"#";
         }
        if (big != null)
        {
             SQL += " and [bid]=" + big + "";
         }
        if (key != null)
        {
             SQL += " and infotext like '%" + key + "%' or aaa   like '%" + key + "%' or title   like '%" + key + "%'";
         }
         OleDbConnection con = new OleDbConnection(connstring);
         OleDbDataAdapter oda = new OleDbDataAdapter();
         oda.SelectCommand = new OleDbCommand(SQL, con);
         DataSet ds = new DataSet();
         oda.Fill(ds,"thc");
        return ds.Tables["thc"];
     }