The DAL layer of the fourth lesson of C# three-tier architecture

The method of the DAL layer is basically the same as that of the BLL layer. This can be done in the early stage, which is simple and easy to understand. In the later stage, a method of BLL may contain several DAL methods.
The reference code is as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Model;
using System.Data.SqlClient;
using System.Data;
//DAL层调用公共的类库
using DbUtil;
using IDAL;

namespace DAL
{
    public class DeptDao:IDept
    {
        //1.复制 service层的几个方法;
        //CRUD:增加  U：修改  D：删除；R:检索;  
        public bool addDept(Dept dept)
        {  
            String sql = "insert dept values(@deptId,@deptName)";

            ////SqlParameter(参数名,值),这里通过封装好的dept对象来取值
            //cmd.Parameters.Add(new SqlParameter("@deptId",dept.DeptId));
            //cmd.Parameters.Add(new SqlParameter("@deptName", dept.DeptName));
            List<SqlParameter> parameters = new List<SqlParameter>();
            //问题转移为如何向集合parameters,增加参数和值;
            parameters.Add(new SqlParameter("@deptId", dept.DeptId));
            parameters.Add(new SqlParameter("@deptName", dept.DeptName));

            return DBHelper.update(sql,parameters);
        }
        public bool updateDept(Dept dept)
        {
            String sql = "update dept set deptName=@deptName where deptId=@deptId";
            List<SqlParameter> parameters = new List<SqlParameter>();
            parameters.Add(new SqlParameter("@deptName",dept.DeptName));
            parameters.Add(new SqlParameter("@deptId",dept.DeptId));

            return DBHelper.update(sql,parameters);
        }
        public bool delDept(Dept dept)           
        {
            String sql = "delete from dept where deptid=@deptId";
            List<SqlParameter> parameters = new List<SqlParameter>();            
            parameters.Add(new SqlParameter("@deptId", dept.DeptId));
            return DBHelper.update(sql, parameters); ;
        }
        public DataTable refreshData() {
            String sql = "select deptId 部门编号,deptName 部门名称 from dept";
            return DBHelper.query(sql,null);
        }
        public DataTable findDeptByName(String deptName)
        {
            String url = "server=.;database=MyDb;uid=sa;pwd=123456";
            //1.链接对象;
            SqlConnection conn = new SqlConnection(url);
            conn.Open();
            //2.SqlCommand
            SqlCommand cmd = conn.CreateCommand();
            //String sql = "select * from dept where deptName like '{0}' ",deptName;
            String sql = "select deptId 部门编号,deptName 部门名称 from dept where deptName like @deptName";
            cmd.CommandText = sql;
            String str = "%" + deptName + "%";
            cmd.Parameters.Add(new SqlParameter("@deptName",str));
            //需要用到适配器对象;适配器没有这种参数方式
            SqlDataAdapter da = new SqlDataAdapter();
            da.SelectCommand = cmd;

            DataTable dt=new DataTable();
            da.Fill(dt);

            return dt;
        }
    }
}