This is an example very very simple and easy of a complete wordpress plugin with table, insert, delete and sub page.
In this example you can manage a Sport Race with Member of each Sport Race. You can also upload photo using the Wordpress Media File Manager. This plugin also expose information with REST API (wp-json), so you can use it on other app via API REST call.
To use it, you must upload file on wp-content/plugins and active it.