C# 数据库查询总结

首先创建了一个SQL Server数据库作为测试的数据库，建立表并填入测试数据

数据库：SQL Server

数据库名：Blog

表名：Test

注：数据库的连接可以使用“dbl”文件测试，具体使用百度即可。

1.将数据库的数据全部查询到datagridview

语句：select * from Test

   private void button1_Click(object sender, EventArgs e)
  {
       string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
       string cmdstr="select * from Test";

       SqlConnection conn = new SqlConnection(connstr);
       conn.Open();

       SqlCommand cmd = new SqlCommand();
       cmd.CommandText = cmdstr;
       cmd.Connection = conn;
       SqlDataAdapter adap = new SqlDataAdapter(cmd);
       DataSet ds = new DataSet();
       adap.Fill(ds);
       dataGridView1.DataSource = ds.Tables[].DefaultView;
  }

2.查询数据库的部分数据并显示到datagridview

语句： string cmdstr = "select 姓名,年龄 from Test";

  private void button2_Click(object sender, EventArgs e)
  {
      string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
      string cmdstr = "select 姓名,年龄 from Test";

      SqlConnection conn = new SqlConnection(connstr);
      conn.Open();

      SqlCommand cmd = new SqlCommand();
      cmd.CommandText = cmdstr;
      cmd.Connection = conn;
      SqlDataAdapter adap = new SqlDataAdapter(cmd);
      DataSet ds = new DataSet();
      adap.Fill(ds);
      dataGridView1.DataSource = ds.Tables[].DefaultView;
  }

3.使用列别名显示数据

语句： string cmdstr = "select 姓名 as name,年龄 as age from Test";

注：语句中的“as”关键词，可以使用空格代替

   private void button3_Click(object sender, EventArgs e)
   {
       string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
       string cmdstr = "select 姓名 as name,年龄 as age from Test";

       SqlConnection conn = new SqlConnection(connstr);
       conn.Open();

       SqlCommand cmd = new SqlCommand();
       cmd.CommandText = cmdstr;
       cmd.Connection = conn;
       SqlDataAdapter adap = new SqlDataAdapter(cmd);
       DataSet ds = new DataSet();
       adap.Fill(ds);
       dataGridView1.DataSource = ds.Tables[].DefaultView;
  }

4.使用列计算

语句：string cmdstr = "select 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test";

   private void button4_Click(object sender, EventArgs e)
   {
      string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
      string cmdstr = "select 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test";

      SqlConnection conn = new SqlConnection(connstr);
      conn.Open();

      SqlCommand cmd = new SqlCommand();
      cmd.CommandText = cmdstr;
      cmd.Connection = conn;
      SqlDataAdapter adap = new SqlDataAdapter(cmd);
      DataSet ds = new DataSet();
      adap.Fill(ds);
      dataGridView1.DataSource = ds.Tables[].DefaultView;
   }

5.限定查询数值范围大小

语句：string cmdstr = "select 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test where (语文+数学+英语)>215";

   private void button5_Click(object sender, EventArgs e)
   {
       string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
       string cmdstr = "select 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test where (语文+数学+英语)>215";

       SqlConnection conn = new SqlConnection(connstr);
       conn.Open();

       SqlCommand cmd = new SqlCommand();
       cmd.CommandText = cmdstr;
       cmd.Connection = conn;
       SqlDataAdapter adap = new SqlDataAdapter(cmd);
       DataSet ds = new DataSet();
       adap.Fill(ds);
       dataGridView1.DataSource = ds.Tables[].DefaultView;
   }

6.通配符

“_”  ：代表一个字符

"%" ：代表多个字符

"[]"  ：代表范围

"[^]”：非括号范围内的单个字符

“distinct”：去除重复记录

7.排序取前2

语句：string cmdstr = "select top 2 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test order by (语文+数学+英语) desc";

   private void button6_Click(object sender, EventArgs e)
   {
       string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
       string cmdstr = "select top 2 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test  order by (语文+数学+英语) desc";

       SqlConnection conn = new SqlConnection(connstr);
       conn.Open();

       SqlCommand cmd = new SqlCommand();
       cmd.CommandText = cmdstr;
       cmd.Connection = conn;
       SqlDataAdapter adap = new SqlDataAdapter(cmd);
       DataSet ds = new DataSet();
       adap.Fill(ds);
       dataGridView1.DataSource = ds.Tables[].DefaultView;
    }

8.排序取前百分之50

语句：string cmdstr = "select top 50 percent 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test order by (语文+数学+英语) desc";

   private void button7_Click(object sender, EventArgs e)
   {
        string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
        string cmdstr = "select top 50 percent 姓名,年龄,语文,数学,英语,语文+数学+英语 as 总分 from Test order by (语文+数学+英语) desc";

        SqlConnection conn = new SqlConnection(connstr);
        conn.Open();

        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = cmdstr;
        cmd.Connection = conn;
        SqlDataAdapter adap = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        adap.Fill(ds);
        dataGridView1.DataSource = ds.Tables[].DefaultView;
   }

9.分组查询、统计

语句：string cmdstr = "select 班级,sum(语文)as 语文总分,sum(数学) as 数学总分,sum(英语) as 英语总分,sum(语文+数学+英语) as 总分 from Test group by 班级";

  private void button8_Click(object sender, EventArgs e)
  {
      string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
      string cmdstr = "select 班级,sum(语文)as 语文总分,sum(数学) as 数学总分,sum(英语) as 英语总分,sum(语文+数学+英语) as 总分 from Test group by 班级";

      SqlConnection conn = new SqlConnection(connstr);
      conn.Open();

      SqlCommand cmd = new SqlCommand();
      cmd.CommandText = cmdstr;
      cmd.Connection = conn;
      SqlDataAdapter adap = new SqlDataAdapter(cmd);
      DataSet ds = new DataSet();
      adap.Fill(ds);
      dataGridView1.DataSource = ds.Tables[].DefaultView;
   }

10.聚合函数

AVG：数字表达式中所有值的平均值

COUNT：选定的行数

MAX：表达式中的最高值

MIN：表达式中的最低值

STDEV：表达式中所有值的统计标准偏差

STDEVP：表达式中所有值的填充统计标准偏差

VAR：表达式中所有值的统计方差

VARP：表达式中所有值的填充统计方差

SUM：数字表达式中所有值的和

11.数据处理函数

LEN：限定字段长度

Ltrim、Rtrim：去处空格函数

left、right：字符函数

ABS：数字函数

Year、Month：日期函数

12.多表分组查询

语句：string cmdstr = "select t.姓名,t.性别,t.年龄,p.name,p.age from Test t,practice p where t.姓名=p.name group by t.姓名,t.性别,t.年龄,p.name,p.age";

注：该功能将同一数据库下的不同数据表进行合并查询，取不同数据表的数据显示到同一个datagridview中

   private void button11_Click(object sender, EventArgs e)
   {
       string connstr = "Data Source=LONG-PC\\LONG;Initial Catalog=Blog;User ID=long;Password=longtao";
       string cmdstr = "select t.姓名,t.性别,t.年龄,p.name,p.age from Test t,practice p  where t.姓名=p.name group by t.姓名,t.性别,t.年龄,p.name,p.age";

       SqlConnection conn = new SqlConnection(connstr);
       conn.Open();

       SqlCommand cmd = new SqlCommand();
       cmd.CommandText = cmdstr;
       cmd.Connection = conn;
       SqlDataAdapter adap = new SqlDataAdapter(cmd);
       DataSet ds = new DataSet();
       adap.Fill(ds);
       dataGridView1.DataSource = ds.Tables[].DefaultView;
   }