sql - 什么使两个表相关？

Question

我是 web 开发 asp.net C# 的新手。我知道给外键(主键 1 表)，然后我们加入这些表。但是当我们加入他们时会发生什么？ (我不知道如何加入表格)我尝试阅读一些文章，但最终没有正确理解它。感谢任何形式的帮助。
这就是我使用两个表的方式。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;

public partial class Result : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand cmd,cmd1;
    SqlDataReader dr;

    string city1, area1, type1, min, max;
    int id = 0;
    int id2 = 0;
    protected void Page_Load(object sender, EventArgs e)
    {
        city1 = Request.QueryString["city"];
        area1 = Request.QueryString["area"];
        type1 = Request.QueryString["propertytype"];
       // type1= "1bhk";
        min = Request.QueryString["minprice"];
        max = Request.QueryString["maxprice"];
        id = Convert.ToInt32(Request.QueryString["uid"]);
       // id = 1;
        con = new SqlConnection("integrated security=true; database=data1; server=sudhir-pc");
        con.Open();
        //cmd = new SqlCommand("select price,area,imagename,users_id from property where city='" + city1 + "' and area='" + area1 + "' and propertytype='" + type1 + "' and users_id=" + id + "", con);
        //  cmd1 = new SqlCommand("select frstname,laststname,contactno from users where users_id='"+id+"'", con);
        cmd = new SqlCommand("select price,area,imagename,users_id,available from property where city=@city1 and area=@area1 and propertytype=@type1", con);
        cmd.Parameters.AddWithValue("@city1",city1);
        cmd.Parameters.AddWithValue("@area1",area1);
        cmd.Parameters.AddWithValue("@type1",type1);

      //  cmd.Parameters.AddWithValue("@id",id);
        dr = cmd.ExecuteReader();

        while (dr.Read())
        {
            Label1.Text = (string)dr["price"].ToString();
            Label2.Text= (string)dr["area"];
            string imgstr=(string)dr["imagename"];
            int id1 = (int)dr["users_id"];
            Image1.ImageUrl = "~/upload/"+imgstr+"";
            Label13.Text = city1;
            Label14.Text = type1;
            Label16.Text = (string)dr["available"].ToString();

            id2 = (int)dr["users_id"];
        }
        dr.Dispose();
        cmd1 = new SqlCommand("select firstname,laststname,contactno from users where users_id=" + id2 + "", con);
        dr = cmd1.ExecuteReader();
        while (dr.Read())
        {
            Label4.Text = (string)dr["firstname"];
            Label5.Text = (string)dr["laststname"];

            if (id > 0)
                Label6.Text = (string)dr["contactno"];


        }
        if (id < 1)
            Label6.Text = "To see contact information. Register!";
    }
}

Answer 1

使用一个外键查询两个表之间的关系示例

"SELECT * FROM Tbl1 as a JOIN Tbl2 as b ON a.PK_tbl1 = b.FK_tbl2"

PK = 主键(Tbl1 的)FK = 外键(Tbl2 的)