我是在做检验用户名不要和数据库的用户名冲突的问题，新手，请大峡们帮忙

这是我自己做的,还没调试,你看看能不能用上.也请各位大侠看看是否有错
protected void Button1_Click(object sender, EventArgs e)
{
if (TextBox1.Text == "" || TextBox2.Text == "")
{
Response.Write("<script defer>alert('请每项都输入！');</script>");
}
//下面就是检查是否有这个用户和是否冲突了
else
{
string strCmd = "select user_name from lab_users where user_ID like'" + TextBox1.Text + "'and password like'" + TextBox2.Text + "'";
SqlConnection myConnection = new SqlConnection(ConfigurationManager.ConnectionStrings["labConnectionString"].ConnectionString);
SqlCommand myCommand = new SqlCommand(strCmd, myConnection);
myCommand.Connection.Open();
string username = (myCommand.ExecuteScalar()).ToString();
myCommand.Connection.Close();
if (username != null)
{
Session["user_ID"] = TextBox1.Text;
Session["user_name"] = username;
string strCmd_type = "select user_type from lab_users where user_ID='" + TextBox1.Text + "'";
SqlCommand myCommand1 = new SqlCommand(strCmd_type, myConnection);
myCommand1.Connection.Open();
int flag = int.Parse(myCommand1.ExecuteScalar().ToString());
if (flag == 0)
{ Response.Redirect("Admin.aspx"); }
else if (flag == 1)
{ Response.Redirect("Leader.aspx"); }
else if (flag == 2)
{ Response.Redirect("A_Teacher.aspx"); }
else
{ Response.Redirect("A_Student.aspx"); }
}
else
{
Response.Write("<script defer>alert('密码不正确！');</script>");
TextBox1.Text = " ";
TextBox2.Text = " ";
}

2楼请加我QQ273085302，问题讨论一下。