c#连接sqlserver数据库

Command对象
作用：向数据库发送SQL语句
封装连接对象和执行的SQL语句
对数据库执行具体的操作，提供增、删、改、查的方法
属性
CommandText 需要封装的SQL语句或存储过程名称
Connection Command对象使用的数据库连接对象
方法
ExecuteNonQuery() 执行增、删、改操作
ExecuteScalar() 返回单一结果的查询 只能返回数据集的第一行第一列 常常用来执行聚合函数
ExecuteReader() 返回只读数据列表的查询

ExecuteNonQuery() 方法使用要点
执行insert update delete类型的语句

执行返回受影响的行数、一般是大于0的整数，0说明没有影响 ，-1表示执行出错

执行增、删、改步骤总结
1 创建Connection对象
2 组合sql语句insert update delete
3 创建Command对象 并封装Connection和sql语句
4 打开连接
5 执行ExecuteNonQuery()方法,返回受影响的行数
6 关闭连接

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    
    class Program
    {
        string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
        SqlConnection conn = null;
        public void OpenDB()
        {
            conn = new SqlConnection(connString);
            if(conn.State==ConnectionState.Closed)
            {
                conn.Open();
            }
            Console.WriteLine("数据库已连接");
        }
        public void CloseDB()
        {
            if(conn.State==ConnectionState.Open)
            {
                conn.Close();
            }
            Console.WriteLine("数据库已断开");
        }
        public int Insert(string sql)
        {
            SqlCommand cmd = new SqlCommand(sql, conn);//简单写法
            //SqlCommand cmd = new SqlCommand
            //{
            //    CommandText = sql,
            //    Connection = conn
            //};
            //ExecuteNonQuery()执行增 删 改 操作
            int result = cmd.ExecuteNonQuery();//返回影响的行数
            return result;
        }
        
        static void Main(string[] args)
        {
            string sql = "insert into student(id,name,age,teacherID) values(5,'小王',25,105)";
            Program p = new Program();
            p.OpenDB();
            int insertResult = p.Insert(sql);
            Console.WriteLine("insertResult:{0}", insertResult);
            p.CloseDB();
            Console.ReadKey();
        }
    }
}

 读取结果集

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    
    class Program
    {
        string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
        public void QueryAllStudents(string sql)
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            while(reader.Read())
            {
                Console.WriteLine("id:{0} name:{1} age:{2} teacherID:{3}", (int)reader["id"], (string)reader["name"], (int)reader["age"], (int)reader["teacherID"]);
            }
            reader.Close();//关闭读取器
            conn.Close();
        }
        
        static void Main(string[] args)
        {
            string sql = "select * from student";
            Program p = new Program();
            p.QueryAllStudents(sql);
            Console.ReadKey();
        }
    }
}

读取多个结果集 在sql server中我们可以这样 select * from student; select * from teacher; 同时执行两条查询语句

在c# 中

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    
    class Program
    {
        string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
        public void QueryAllStudents(string sql)
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            while(reader.Read())
            {
                Console.WriteLine("id:{0} name:{1} age:{2} teacherID:{3}", (int)reader["id"], (string)reader["name"], (int)reader["age"], (int)reader["teacherID"]);
            }
            if(reader.NextResult())
            {
                while(reader.Read())
                {
                    Console.WriteLine("id:{0} name:{1} major:{2}", reader["id"], reader["name"], reader["major"]);
                }
            }
            reader.Close();//关闭读取器
            conn.Close();
        }
        
        static void Main(string[] args)
        {
            string sql = "select * from student;select * from teacher";
            Program p = new Program();
            p.QueryAllStudents(sql);
            Console.ReadKey();
        }
    }
}

 实现一个简单的SQL Helper类

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    class SQLHelper
    {
        private static string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
        public static int GetSingleQuery(string sql)//一般用于聚合函数的查询
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            int result = (int)cmd.ExecuteScalar();
            conn.Close();
            return result;
        }
        public static int Update(string sql)
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            int result = cmd.ExecuteNonQuery();//返回影响的行数
            conn.Close();
            return result;
        }
        public static SqlDataReader GetReader(string sql)
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            //SqlDataReader reader = cmd.ExecuteReader();
            //reader.Close();
            //conn.Close(); //这样写会报阅读器关闭时尝试调用 Read 无效错误
            //解决方法
            SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);//关闭读取器时自动关闭connection连接
            return reader;
        }
        static void Main(string[] args)
        {
            //测试代码
            //string sql = "select * from student";
            //SqlDataReader reader = GetReader(sql);
            //while(reader.Read())
            //{
            //    Console.WriteLine("id:{0} name:{1} age:{2} teacherID:{3}", (int)reader["id"], reader["name"].ToString(), (int)reader["age"], (int)reader["teacherID"]);
            //}
            //reader.Close();//关闭读取器

            //string sql = "select count(*) from student";
            //int result = GetSingleQuery(sql);
            //Console.WriteLine("result stu count:{0}", result);

            //string sql = "update student set name='小李' where name='小王'";
            //int result = Update(sql);
            //Console.WriteLine("update result:{0}", result);
            Console.ReadKey();
        }
    }
}
将查询到的数据以对象的形式返回

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    class Test
    {
        private static string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
        public Student QueryStuByID(string sql)
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            Student stu = null; 
            if(reader.Read())
            {
                stu = new Student
                {
                    Id = (int)reader["id"],
                    Name = reader["name"].ToString(),
                    Age = (int)reader["age"],
                    TeacherID = (int)reader["teacherID"]
                };
            }
            reader.Close();
            conn.Close();
            return stu;
        }
        static void Main(string[] args)
        {
            //测试代码
            //string sql = "select * from student where id=3";
            //Test t = new Test();
            //Student stu = t.QueryStuByID(sql);
            //if(stu!=null)
            //{
            //    Console.WriteLine("stu name:{0}", stu.Name);
            //}
            Console.ReadKey();
        }
    }
    class Student
    {
        private int id;
        private string name;
        private int age;
        private int teacherID;

        public int Id { get => id; set => id = value; }
        public string Name { get => name; set => name = value; }
        public int Age { get => age; set => age = value; }
        public int TeacherID { get => teacherID; set => teacherID = value; }
    }
}

将查询到的数据以对象集合的方式返回 和上述代码类似

连表查询

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PracticeFive
{
    class Test
    {
        private static string connString = "server=DESKTOP-BQDF638;database=test_four;uid=sa;pwd=123456";
 
        public List<StudentTeacher> UnionQuery(string sql)//联合查询
        {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            List<StudentTeacher> lstStuTea = new List<StudentTeacher>();
            while(reader.Read())
            {
                StudentTeacher stuTea = new StudentTeacher();
                stuTea.Stu = new Student
                {
                    Name = reader["stuName"].ToString()
                };
                stuTea.Tea = new Teacher
                {
                    Name = reader["teaName"].ToString(),
                    Major = reader["major"].ToString()
                };
                lstStuTea.Add(stuTea);
            }
            reader.Close();
            conn.Close();
            return lstStuTea;
        }
        static void Main(string[] args)
        {
            //因为学生表和老师表都有name字段 计算机无法区分 所有取别名来区分
            string sql = "select a.name as stuName,b.name as teaName,b.major from student a inner join teacher b on a.teacherID=b.id";
            Test test = new Test();
            List<StudentTeacher> lst = test.UnionQuery(sql);
            foreach(var item in lst)
            {
                Console.WriteLine("student name:{0} teacher name:{1} teacher major:{2}", item.Stu.Name, item.Tea.Name, item.Tea.Major);
            }
            Console.ReadKey();
        }
    }
    class Student
    {
        private int id;
        private string name;
        private int age;
        private int teacherID;

        public int Id { get => id; set => id = value; }
        public string Name { get => name; set => name = value; }
        public int Age { get => age; set => age = value; }
        public int TeacherID { get => teacherID; set => teacherID = value; }
    }
    class Teacher
    {
        private int id;
        private string name;
        private string major;

        public int Id { get => id; set => id = value; }
        public string Name { get => name; set => name = value; }
        public string Major { get => major; set => major = value; }
    }
    class StudentTeacher//联合查询时使用
    {
        private Student stu;
        private Teacher tea;

        internal Student Stu { get => stu; set => stu = value; }
        internal Teacher Tea { get => tea; set => tea = value; }
    }
}