增删改查练习

1. 改

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

namespace UpdatingData
{
    class Program
    {
        static void Main(string[] args)
        {
            string connstring = @"Server=localhost;Integrated Security=True; Database=northwind";
            string strsql = @"select customerID,CompanyName from Customers";

            SqlConnection thisConnection = new SqlConnection(connstring);
            SqlDataAdapter thisAdapter = new SqlDataAdapter(strsql, thisConnection);
            SqlCommandBuilder thisBuilder = new SqlCommandBuilder(thisAdapter); // SqlCommandBuilder

            DataSet thisDataSet = new DataSet();
            thisAdapter.Fill(thisDataSet, "Customers");

            Console.WriteLine("name before change:{0}", thisDataSet.Tables["Customers"].Rows[9]["CompanyName"]);
            thisDataSet.Tables["Customers"].Rows[9]["CompanyName"] = "Acme,Inc.";

            thisAdapter.Update(thisDataSet, "Customers");//Update

            Console.WriteLine("name after change:{0}", thisDataSet.Tables["Customers"].Rows[9]["CompanyName"]);

            thisConnection.Close();
            Console.WriteLine("Program finished, press Enter/Return to continue:");
            Console.ReadLine();
        }
    }
}

CommandBuilder:命令构造器
CommandBuilder最大的用处就是组建一个DataAdapter的UpdateCommand,InsertCommand,DeleteCommand这三个Command的CommandText。

一般来说，当使用DataAdapter的Update方法时，DataAdapter都会把相应的工作分给上述三个Command来处理，也就是说，要用Update方法，就必须配置好这三个Command。
如果我们用手工去配置的话，那工作量是很惊人的，而且还不一定正确。所以MS给我们提供了一个专门的工具CommandBuilder。
如果没有SqlCommandBuilder thisBuilder = new SqlCommandBuilder(thisAdapter); 这一句, 那么, thisAdapter.Update(thisDataSet, "Customers");就会报错.
2.增

namespace AddingData
{
    class Program
    {
        static void Main(string[] args)
        {

            string connstring = @"Server=localhost;Integrated Security=True; Database=northwind";
            string strsql = @"select customerID,CompanyName from Customers";
            SqlConnection thisConnection = new SqlConnection(connstring);
            SqlDataAdapter thisAdapter = new SqlDataAdapter(strsql, thisConnection);

            SqlCommandBuilder thisBuilder = new SqlCommandBuilder(thisAdapter);
            DataSet thisDataSet = new DataSet();
            thisAdapter.Fill(thisDataSet, "Customers");

            Console.WriteLine("# rows before change:{0}", thisDataSet.Tables["Customers"].Rows.Count);

            DataRow thisrow = thisDataSet.Tables["Customers"].NewRow();
            thisrow["CustomerID"] = "ZACZI";
            thisrow["CompanyName"] = "Zachary Zithers Ltd.";
            thisDataSet.Tables["Customers"].Rows.Add(thisrow);

            Console.WriteLine("# rows after change:{0}", thisDataSet.Tables["Customers"].Rows.Count);

            thisAdapter.Update(thisDataSet, "Customers");

            thisConnection.Close();
            Console.WriteLine("Program finished, press Enter/Return to continue:");
            Console.ReadLine();
        }
    }
}

DataSet是内存中非连接的数据副本, DataAdapter负责连接到磁盘上的数据库, 因此需要调用它的Update()方法, 才能使DataSet中的内存数据与磁盘上的数据库同步.
3. 查

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

namespace FindingData
{
    class Program
    {
        static void Main(string[] args)
        {
            string connstring = @"Server=localhost;Integrated Security=True; Database=northwind";
            string strsql = @"select customerID,CompanyName from Customers";
            SqlConnection thisConnection = new SqlConnection(connstring);

            SqlDataAdapter thisAdapter = new SqlDataAdapter(strsql, thisConnection);
            SqlCommandBuilder thisBuilder = new SqlCommandBuilder(thisAdapter);

            DataSet thisDataSet = new DataSet();
            thisAdapter.Fill(thisDataSet, "Customers");

            Console.WriteLine("# rows before change:{0}", thisDataSet.Tables["Customers"].Rows.Count);

            DataColumn[] keys = new DataColumn[1];   //创建一个DataColumns数组
            keys[0] = thisDataSet.Tables["Customers"].Columns["CustomerID"];  //将键数组的第一个元素keys[0]赋给Customers表中的CustomerID列. 
            thisDataSet.Tables["Customers"].PrimaryKey = keys;  //将keys赋给DataTable对象Customers的PrimaryKey属性

            //thisAdapter.MissingMappingAction = MissingSchemaAction.AddWithKey;
            //thisAdapter.Fill(thisDataSet, "Customers");  //书上说这样也可以,但是运行出错:不能隐式转换

            DataRow findRow = thisDataSet.Tables["Customers"].Rows.Find("ZACZI");  //因为Find()返回一个DataRow,因此建立一个DataRow对象findRow来获取结果

            if (findRow == null)     
            {
                Console.WriteLine("ZACZI not found, will add to Customers table");
                DataRow thisRow = thisDataSet.Tables["Customers"].NewRow();
                thisRow["CustomerID"] = "ZACZI";
                thisRow["CompanyName"] = "Zachary Zithers Ltd.";
                thisDataSet.Tables["Customers"].Rows.Add(thisRow);

                if ((findRow = thisDataSet.Tables["Customers"].Rows.Find("ZACZI")) != null)
                {
                    Console.WriteLine("ZACZI successfully added to Customers table");
                }
            }
            else
            {
                Console.WriteLine("ZACZI already present in database");
            }

            thisAdapter.Update(thisDataSet, "Customers");

            Console.WriteLine("# rows after change {0}", thisDataSet.Tables["Customers"].Rows.Count);

            thisConnection.Close();
            Console.WriteLine("Program finished, press Enter/Return to continue:");
            Console.ReadLine();
        }
    }
}

使用Find()检查要添加的行是否已经存在.
在使用Find()之前, 需要构建一个主键. 主键就是用于搜索的键. 它由表的一列或多列构成, 包含可以在表中唯一标识行的值或值的集合.
因此, 当通过键搜索时, 可以找到唯一的一行.
4.删

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;

namespace DeletingData
{
    class Program
    {
        static void Main(string[] args)
        {
            string connstring = @"Server=localhost;Integrated Security=True; Database=northwind";
            string strsql = @"select customerID,CompanyName from Customers";
            SqlConnection thisConnection = new SqlConnection(connstring);

            SqlDataAdapter thisAdpter = new SqlDataAdapter(strsql, thisConnection);

            SqlCommandBuilder thisBuilder = new SqlCommandBuilder(thisAdpter);

            DataSet thisDataSet = new DataSet();
            thisAdpter.Fill(thisDataSet, "Customers");

            Console.WriteLine("# rows before change:{0}", thisDataSet.Tables["Customers"].Rows.Count);

            DataColumn[] keys = new DataColumn[1];
            keys[0] = thisDataSet.Tables["Customers"].Columns["CustomerID"];
            thisDataSet.Tables["Customers"].PrimaryKey = keys;

            DataRow findRow = thisDataSet.Tables["Customers"].Rows.Find("ZACZI");

            if (findRow != null)
            {
                Console.WriteLine("ZACZI already in Customers table");
                Console.WriteLine("Removing ZACZI");

                findRow.Delete();
                thisAdpter.Update(thisDataSet, "Customers");
            }

            Console.WriteLine("# rows after change: {0}", thisDataSet.Tables["Customers"].Rows.Count);

            thisConnection.Close();
            Console.WriteLine("Program finished, press Enter/Return to continue:");
            Console.ReadLine();
        }
    }
}

当调用Delete()时, 在调用Update改变数据库之前, 数据库中的行是不会被删除的.
实际上, Delete()方法并不执行删除操作, 它仅仅标记要删除的行.