求助：Oracle查询问题

同一组数据，用参数化查询，返回错误0，拼接字符串，返回正确值1。不知何故？

public bool IsMeetRzlx2(string rzlx, string item, string wlbm, string gysh)
{
    string sql = @"SELECT COUNT(0) FROM ssaln.ttcibd924400 b
WHERE b.t$suno=:v_gysh
AND b.t$item=:v_item
AND b.t$rzlx=:v_rzlx
AND b.t$mitm=:v_mitm";
    OracleParameter[] _param = {
                                   new OracleParameter("v_gysh", gysh),
                                   new OracleParameter("v_item", wlbm),
                                   new OracleParameter("v_rzlx", rzlx),
                                   new OracleParameter("v_mitm", item)
                               };
    OracleConnection conn = new OracleConnection("…");
    OracleCommand cmd=  new OracleCommand(sql, conn);
    cmd.Parameters.AddRange(_param);
    conn.Open();
    object obj = cmd.ExecuteScalar();
    conn.Close();
    return (obj != DBNull.Value && Convert.ToInt32(obj) > 0) ? true : false;
}

public bool IsMeetRzlx2(string rzlx, string item, string wlbm, string gysh)
{
    string sql = "SELECT COUNT(0) FROM ssaln.ttcibd924400 b "
+ "WHERE b.t$suno='" + gysh + "' "
+ "AND b.t$item='" + wlbm + "' "
+ "AND b.t$rzlx='" + rzlx + "' "
+ "AND b.t$mitm='" + item + "'"; 

    OracleConnection conn = new OracleConnection("…");
    OracleCommand cmd=  new OracleCommand(sql, conn);
    conn.Open();
    object obj = cmd.ExecuteScalar();
    conn.Close();
    return (obj != DBNull.Value && Convert.ToInt32(obj) > 0) ? true : false;
}