программа
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace LR3
{
    static class Program
    {
        /// <summary>
        /// Главная точка входа для приложения.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
        }
    }
}

основное окно
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

using LR3.Controller;

namespace LR3
{
    public partial class MainForm : Form
    {
        Query controller;

        public MainForm()
        {
            InitializeComponent();
            controller = new Query(ConnectionString.ConnStr);
            DataGrid.DataSource = controller.UpdateOperation();
        }

        private void UpdateButton_Click(object sender, EventArgs e)
        {
            DataGrid.DataSource = controller.UpdateOperation();
        }

        private void AddButton_Click(object sender, EventArgs e)
        {
            Form dialog_window = new AddNewForm();
            dialog_window.ShowDialog();
            DataGrid.DataSource = controller.UpdateOperation();
        }

        private void DeleteButton_Click(object sender, EventArgs e)
        {
            controller.Delete(int.Parse(DataGrid.Rows[DataGrid.CurrentRow.Index].Cells["ID"].Value.ToString()));
            DataGrid.DataSource = controller.UpdateOperation();
        }

        private void GenerateReportButton_Click(object sender, EventArgs e)
        {
            Form newForm = new GenerateReportForm();
            newForm.Show();
        }
    }
}

форма добавления операции
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

using LR3.Controller;

namespace LR3
{
    public partial class AddNewForm : Form
    {
        bool CategoryTextBoxClicked = false;
        bool TypeTextBoxClicked = false;
        bool SumTextBoxClicked = false;
        bool OpDateTimeTextBoxClicked = false;
        bool ComissionTextBoxClicked = false;

        Query controller;

        public AddNewForm()
        {
            InitializeComponent();
            controller = new Query(ConnectionString.ConnStr);
        }

        private void AddButton_Click(object sender, EventArgs e)
        {
            controller.Add(CategoryTextBox.Text,
                TypeTextBox.Text,
                decimal.Parse(SumTextBox.Text),
                DateTime.Parse(OpDateTimeTextBox.Text),
                decimal.Parse(ComissionTextBox.Text));
            this.Close();
        }

        private void CategoryTextBox_Click(object sender, EventArgs e)
        {
            if (!CategoryTextBoxClicked)
            {
                CategoryTextBoxClicked = true;
                CategoryTextBox.Text = string.Empty;
            }
        }

        private void TypeTextBox_Click(object sender, EventArgs e)
        {
            if (!TypeTextBoxClicked)
            {
                TypeTextBoxClicked = true;
                TypeTextBox.Text = string.Empty;
            }
        }

        private void SumTextBox_Click(object sender, EventArgs e)
        {
            if (!SumTextBoxClicked)
            {
                SumTextBoxClicked = true;
                SumTextBox.Text = string.Empty;
            }
        }

        private void OpDateTimeTextBox_Click(object sender, EventArgs e)
        {
            if (!OpDateTimeTextBoxClicked)
            {
                OpDateTimeTextBoxClicked = true;
                OpDateTimeTextBox.Text = string.Empty;
            }
        }

        private void ComissionTextBox_Click(object sender, EventArgs e)
        {
            if (!ComissionTextBoxClicked)
            {
                ComissionTextBoxClicked = true;
                ComissionTextBox.Text = string.Empty;
            }
        }
    }
}

форма отчета
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

using LR3.Controller;

namespace LR3
{
    public partial class ResultForm : Form
    {
        Query controller;

        public ResultForm()
        {
            InitializeComponent();
            controller = new Query(ConnectionString.ConnStr);
        }
        public ResultForm(DateTime dateFrom, DateTime dateTo, bool isCategory, string categoryOrType)
        {
            InitializeComponent();
            controller = new Query(ConnectionString.ConnStr);

            DateFromTextBox.Text = dateFrom.ToString();
            DateToTextBox.Text = dateTo.ToString();

            string label = string.Empty;
            DataTable data;
            if(isCategory)
            {
                label += "Category: ";
                data = controller.DoCategoryStuff(dateFrom, dateTo, categoryOrType);
            } else {
                label += "Type: ";
                data = controller.DoTypeStuff(dateFrom, dateTo, categoryOrType);
            }
            label += categoryOrType;
            CategoryOrTypeLabel.Text = label;

            DataGrid.DataSource = data;

            OpCountTextBox.Text = data.Rows.Count.ToString();

            decimal totalSum = 0;

            foreach(DataRow row in data.Rows)
            {
                totalSum += row.Field<decimal>("Sum");
            }

            AvOpSumTextBox.Text = (totalSum / data.Rows.Count).ToString();
        }
    }
}

запросы
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.OleDb;
using System.Data;

namespace LR3.Controller
{
    class Query
    {
        OleDbConnection     connection;
        OleDbCommand        command;
        OleDbDataAdapter    dataAdaptor;
        DataTable           bufferTable;

        public Query(string ConnStr)
        {
            connection = new OleDbConnection(ConnStr);
            bufferTable = new DataTable();
        }
        public DataTable UpdateOperation()
        {
            connection.Open();
            dataAdaptor = new OleDbDataAdapter("SELECT * FROM Operation", connection);

            bufferTable.Clear();
            dataAdaptor.Fill(bufferTable);

            connection.Close();
            return bufferTable;
        }

        public void Add(string Category, string Type, decimal Sum, DateTime OpDateTime, decimal Comission)
        {
            connection.Open();

            command = new OleDbCommand($"INSERT INTO Operation([ID], [Category], [Type], [Sum], [OpDateTime], [Comission]) VALUES(@ID, @Category, @Type, @Sum, @OpDateTime, @Comission)", connection);
            command.Parameters.Add("@ID", OleDbType.Integer).Value = GetNextID();
            command.Parameters.Add("@Category", OleDbType.VarChar).Value = Category;
            command.Parameters.Add("@Type", OleDbType.VarChar).Value = Type;
            command.Parameters.Add("@Sum", OleDbType.Currency).Value = Sum.ToString("0.0000");
            command.Parameters.Add("@OpDateTime", OleDbType.Date).Value = OpDateTime;
            command.Parameters.Add("@Comission", OleDbType.Currency).Value = Comission.ToString("0.0000");

            command.ExecuteNonQuery();

            connection.Close();
        }

        private int GetNextID()
        {
            dataAdaptor = new OleDbDataAdapter("SELECT MAX([ID]) AS [ID] FROM Operation", connection);

            bufferTable.Clear();
            dataAdaptor.Fill(bufferTable);


            var row = bufferTable.Rows[0];
            int res = row.IsNull("ID") ? 0 : bufferTable.Rows[0].Field<int>("ID") + 1;

            return res;
        }

        public void Delete(int ID)
        {
            connection.Open();

            command = new OleDbCommand($"DELETE FROM Operation WHERE ID = {ID}", connection);
            command.ExecuteNonQuery();

            connection.Close();
        }

        public DataTable DoCategoryStuff(DateTime dateFrom, DateTime dateTo, string category)
        {
            connection.Open();
            command = new OleDbCommand("SELECT [Type], [Sum], [OpDateTime], [Comission] FROM Operation WHERE [Category] = @category AND [OpDateTime] BETWEEN @dateFrom AND @dateTo ORDER BY [Comission]", connection);

            command.Parameters.Add("@category", OleDbType.VarChar).Value = category;
            command.Parameters.Add("@dateFrom", OleDbType.Date).Value = dateFrom;
            command.Parameters.Add("@dateTo", OleDbType.Date).Value = dateTo;

            dataAdaptor = new OleDbDataAdapter(command);

            string testString = command.CommandText;

            bufferTable.Clear();
            dataAdaptor.Fill(bufferTable);

            connection.Close();
            return bufferTable;
        }

        public DataTable DoTypeStuff(DateTime dateFrom, DateTime dateTo, string type)
        {
            connection.Open();
            command = new OleDbCommand("SELECT [Category], [Sum], [OpDateTime], [Comission] FROM Operation WHERE [Type] = @type AND [OpDateTime] BETWEEN @dateFrom AND @dateTo ORDER BY [Comission]", connection);

            command.Parameters.Add("@type", OleDbType.VarChar).Value = type;
            command.Parameters.Add("@dateFrom", OleDbType.Date).Value = dateFrom;
            command.Parameters.Add("@dateTo", OleDbType.Date).Value = dateTo;

            dataAdaptor = new OleDbDataAdapter(command);

            bufferTable.Clear();
            dataAdaptor.Fill(bufferTable);

            connection.Close();
            return bufferTable;
        }
    }
}