Update transaction's code

lab15-library-management-system/README.md

@@ -6,3 +6,42 @@
 
 [mindmap](https://arronvague.github.io/Database-Systems-Laboratory/lab15-library-management-system/mindmap/mindmap.html)
 
+**事务**
+
+```csharp
+using (MySqlConnection connection = Database.GetMySqlConnection())
+{
+    connection.Open();
+
+    MySqlTransaction transaction = connection.BeginTransaction();
+
+    try
+    {
+        // 设置事务关联的命令
+        ...
+
+        // 执行事务关联的命令
+        command1.ExecuteNonQuery();
+        command2.ExecuteNonQuery();
+        command3.ExecuteNonQuery();
+        command4.ExecuteNonQuery();
+
+        // 提交事务
+        transaction.Commit();
+        Console.WriteLine("借阅图书成功！");
+
+        ClearTextBox();
+        DataBind_Customer();
+        lbl_Note.Text = "Borrowed successfully!";
+    }
+    catch (Exception ex)
+    {
+        // 回滚事务
+        transaction.Rollback();
+        Console.WriteLine("借阅图书失败！");
+        Console.WriteLine("错误信息：" + ex.Message);
+    }
+
+}
+```
+