ClassId,ClassName,GradeName from ClassInfo c,GradeInfo g where c.GradeId=g.GradeId";
DataTable...= DBNull.Value && ((int)o) > 0)
{
MessageBox.Show("该学生已存在!"...ClassId,ClassName,GradeName from ClassInfo c,GradeInfo g where c.GradeId=g.GradeId";
DataTable...ClassId,ClassName,GradeName from ClassInfo c,GradeInfo g where c.GradeId=g.GradeId";
DataTable...= DBNull.Value && ((int)o) > 0)
{
MessageBox.Show("该学生已存在,请重新修改!"