C#实现SqlServer连接查询

最近的工作遇到了连接查询，特在此记录，以免日后遗忘。以前学SQL连接查询的时候，通篇都是理论和数学公式，既抽象又不便于理解，因此我在这里以一个最通俗易懂的例子来说明。首先在SqlServer中建立两个表：TProvince和TCity，前者记录省，后者记录市，其数据如下所示：

Id	ProvinceName
1	浙江省
2	江苏省
3	安徽省
4	广东省

Id	CityName	ProvinceId
1	杭州市	        1
2	湖州市	        1
3	嘉兴市	        1
4	南京市	        2
5	苏州市	        2
6	无锡市	        2
7	合肥市	        3
8	芜湖市	        3
9	六安市	        3
10	济南市	        5

内连接

// 内连接
private void button1_Click(object sender, EventArgs e)
{
    string commandText = "select * from [TCity] inner join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
    using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
    {
        DataTable dataTable = new DataTable();
        adapter.Fill(dataTable);
        dataGridView1.DataSource = dataTable;
    }
}

从结果可以看出，内连接就是获取两张表能够对应上的数据，因此广东省和济南市这两条记录不会显示。

左连接

// 左连接
private void button2_Click(object sender, EventArgs e)
{
    string commandText = "select * from [TCity] left join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
    using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
    {
        DataTable dataTable = new DataTable();
        adapter.Fill(dataTable);
        dataGridView1.DataSource = dataTable;
    }
}

从结果可以看出，左连接确保了市的名称全部显示，但广东省由于没有对应的市，因此没有显示。因此左连接其实就是确保左边数据表记录的完整，右边则不去管它。

右连接

// 右连接
private void button3_Click(object sender, EventArgs e)
{
    string commandText = "select * from [TCity] right join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
    using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
    {
        DataTable dataTable = new DataTable();
        adapter.Fill(dataTable);
        dataGridView1.DataSource = dataTable;
    }
}

从结果可以看出，右连接确保了右边数据表的完整，而左边则不去管它。

全连接

// 全连接
private void button4_Click(object sender, EventArgs e)
{
    string commandText = "select * from [TCity] full join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
    using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
    {
        DataTable dataTable = new DataTable();
        adapter.Fill(dataTable);
        dataGridView1.DataSource = dataTable;
    }
}

从结果可以看出，全连接确保了左右两侧数据表的完整。

交叉连接

// 交叉连接
private void button5_Click(object sender, EventArgs e)
{
    string commandText = "select * from [TCity] cross join [TProvince]";
    using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
    {
        DataTable dataTable = new DataTable();
        adapter.Fill(dataTable);
        dataGridView1.DataSource = dataTable;
    }
}

从结果可以看出，交叉连接就不管对应不对应了，它只是左边数据与右边数据的逐一匹配，也就是笛卡尔积。全部代码如下所示：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        // 连接字符串
        private static string ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();

        // 构造函数
        public Form1()
        {
            InitializeComponent();
        }

        // 内连接
        private void button1_Click(object sender, EventArgs e)
        {
            string commandText = "select * from [TCity] inner join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
            using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
            {
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);
                dataGridView1.DataSource = dataTable;
            }
        }

        // 左连接
        private void button2_Click(object sender, EventArgs e)
        {
            string commandText = "select * from [TCity] left join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
            using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
            {
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);
                dataGridView1.DataSource = dataTable;
            }
        }

        // 右连接
        private void button3_Click(object sender, EventArgs e)
        {
            string commandText = "select * from [TCity] right join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
            using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
            {
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);
                dataGridView1.DataSource = dataTable;
            }
        }

        // 全连接
        private void button4_Click(object sender, EventArgs e)
        {
            string commandText = "select * from [TCity] full join [TProvince] on [TCity].ProvinceId=[TProvince].Id";
            using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
            {
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);
                dataGridView1.DataSource = dataTable;
            }
        }

        // 交叉连接
        private void button5_Click(object sender, EventArgs e)
        {
            string commandText = "select * from [TCity] cross join [TProvince]";
            using (SqlDataAdapter adapter = new SqlDataAdapter(commandText, ConnectionString))
            {
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);
                dataGridView1.DataSource = dataTable;
            }
        }
    }
}