string kk = "tt,oo,pp";    //假设从文本框获取的值是字符串kk
        string[] b = kk.Split(',');    //将字符串中的","除去后存入数组里
        string endstr = "";
        for (int i = 0; i < b.Length; i++)  //根据数组的元素个数判断循环次数
        {
            kk = "'" + b[i] + "'";  //在每个元素前后加上我们想要的格式，效果例如：
            // " 'tt' "
            if (i < b.Length - 1)  //根据数组元素的个数来判断应该加多少个逗号
            {
                kk += ",";
            }
            endstr += kk;
        }
        string sqlstr = "select * from tablename where xxx in(" + endstr + ")";
   //最后 select * from tablename where xxx in ('tt','oo','pp')