关于SQL(asp),该怎么解决

关于SQL(asp)
can_s=request.QueryString( "zi_y ")
    sql= "select   *   from   xin_x_l_b   where   Lei_b_bh= "&can_s
如果zi_y是数字型时（长整型）时我发现这句话没有问题
但是zi_y是字符型时就错，
老师，这是为什么呢？


[解决办法]
sql= "select * from xin_x_l_b where Lei_b_bh= ' "&can_s& " ' "