C# connects MySQL to make dataGridView pagination

foreword

Recently, I am developing a software, C# connects to the MySQL database, uses the dataGridView control in the software, and then adds a page under the dataGridView control. At the beginning, I found some C# pagination controls, and tried to introduce the project, but there were more or less regrets. So I tried to write this pagination myself.

I stepped on a lot of pitfalls in the process, and now I will write it out and break it down.

1. Software interface

Pagination consists of two Buttons, a TextBox and a Label.

2. Core code

1. Global variables and load functions

        int pagenum;//页码
        int zts;//总条数
        public void pageLode()
        {
    
    
            pagenum = 1;
            txt页码.Text = pagenum.ToString();
            selectData();
            lab总页.Text = (zts / 15 + 1).ToString();
            btn上一页.Enabled = false;
            btn下一页.Enabled = true;
        }

The pageLode() function needs to be placed in the form Load() method.

		private void Manager_Load(object sender, EventArgs e)
		{
    
    
			pageLode();
		}

2. Calculate the page number

        public void selectData()
        {
    
    
            string conn = myDatabase.Common;
            MySqlConnection mycon = new MySqlConnection(conn);
            try
            {
    
    
                if (mycon.State == System.Data.ConnectionState.Closed)
                {
    
    
                    mycon.Open();
                }
                string sql_count = "SELECT COUNT(uid) as tally FROM code_custom";
                MySqlCommand cmd_count = new MySqlCommand(sql_count, mycon);
                MySqlDataReader sread = cmd_count.ExecuteReader();
                sread.Read();
                zts = Convert.ToInt32(sread["tally"]);
            }
            catch (MySqlException ex)
            {
    
    
                MessageBox.Show("不能连接服务器，本地验证失败！" + ex.Message);
            }
            finally
            {
    
    
                mycon.Close();
            }         
        }

The data requested by the url network used here.

3. Previous page

        private void Btn上一页_Click(object sender, EventArgs e)
        {
    
    
            btn下一页.Enabled = true;
            pagenum = pagenum - 1;
            txt页码.Text = pagenum.ToString();
            if (pagenum <= 1)
            {
    
    
                txt页码.Text = "1";
                btn上一页.Enabled = false;
                btn重新载入.Focus();
            }
            selectPage(pagenum);
        }

4. Next page

        private void Btn下一页_Click(object sender, EventArgs e)
        {
    
    
            btn上一页.Enabled = true;
            pagenum = pagenum + 1;
            txt页码.Text = pagenum.ToString();

            if (pagenum == Convert.ToInt32(lab总页.Text))
            {
    
    
                txt页码.Text = lab总页.Text;
                btn下一页.Enabled = false;
                btn重新载入.Focus();
    
            }
            selectPage(pagenum);
        }

5. Read data in pages

        public void selectPage(int p)
        {
    
    
            string fenye = (15 * (p - 1)).ToString();            
            string conn = myDatabase.Common;
            MySqlConnection mycon = new MySqlConnection(conn);
            try
            {
    
    
                if (mycon.State == System.Data.ConnectionState.Closed)
                {
    
    
                    mycon.Open();
                }
                dataGridView1.Rows.Clear();
                string sql_uid = "SELECT * FROM code_custom WHERE uid NOT IN (SELECT uid FROM (SELECT uid FROM code_custom ORDER BY uid desc LIMIT " + fenye + " ) as cd ) ORDER BY uid desc LIMIT 15";

                MySqlCommand cmd_uid = new MySqlCommand(sql_uid, mycon);
                MySqlDataReader reader = cmd_uid.ExecuteReader();
                while (reader.Read())
                {
    
    
                    int index = this.dataGridView1.Rows.Add();
                    this.dataGridView1.Rows[index].Cells[1].Value = reader.GetString("uid");
                    this.dataGridView1.Rows[index].Cells[2].Value = reader.GetString("ucode");
                    ···
                    ···
                    ···
                    ···
                }
            }
            catch (MySqlException ex)
            {
    
    
                MessageBox.Show("不能连接服务器，本地验证失败！" + ex.Message);
            }
            finally
            {
    
    
                mycon.Close();
            }

        }

---

part of stepping on the pit

The most time-consuming part of this part of the code is writing SQL statements. I used to refer to the SQL statement written by MSSQL, but later found that MySQL is different from MSSQL in many places.
For example, there is such a syntax in MSSQL:

SELECT TOP 15 * FROM table_name;

But there is no such syntax as "TOP 15" in MySQL, so it needs to be written like this in MySQL:

SELECT * FROM table_name LIMIT 15;

Secondly, in MySQL syntax, LIMIT is not allowed in subqueries , such as:

SELECT * FROM table_name WHERE uid NOT IN 
	(SELECT uid FROM code_custom ORDER BY uid desc LIMIT 30) ORDER BY uid desc LIMIT 15;

Such a statement is wrong in MySQL, so such a prompt will appear.

After optimization, you need to add an alias to the subquery. So the correct code is as follows:

SELECT * FROM code_custom WHERE uid NOT IN 
	(SELECT uid FROM 
		(SELECT uid FROM code_custom ORDER BY uid desc LIMIT " + fenye + " ) as cd ) 
ORDER BY uid desc LIMIT 15"

To sum up, I am still not very proficient in mastering SQL statements.