private void ALLBT_Click(object sender, EventArgs e)
 {
     if (ConnStatus == true)
     {
         MainDGV.DataSource = DB_Handle.Reader("SELECT * FROM Invoices").Tables[0];
     }
     else
     {
         MessageBox.Show("DB connection not establised.");
     }
 }
 private void SearchBT_Click(object sender, EventArgs e)
 {
     if (ConnStatus == true)
     {
         MainDGV.DataSource = DB_Handle.Reader(
             $"SELECT * FROM Invoices WHERE Client LIKE " +
             $"'%{Interaction.InputBox("Please input FIO or part of FIO.", "Search by FIO")}%'").Tables[0];
     }
     else
     {
         MessageBox.Show("DB connection not establised.");
     }
 }
 private void TotAmoBT_Click(object sender, EventArgs e)
 {
     if (ConnStatus == true)
     {
         MainDGV.DataSource = DB_Handle.Reader(
             "SELECT " +
             "CONCAT((SELECT MIN(date) FROM TestTaskInvoiceDB.dbo.Invoices), ' - '," +
             "(SELECT MAX(date) FROM TestTaskInvoiceDB.dbo.Invoices)) AS 'Period'," +
             "SUM(Invoice_amount) AS 'Total Amount'" +
             "FROM TestTaskInvoiceDB.dbo.Invoices"
             ).Tables[0];
         MainDGV.Columns[0].AutoSizeMode = DataGridViewAutoSizeColumnMode.DisplayedCells;
     }
     else
     {
         MessageBox.Show("DB connection not establised.");
     }
 }