Exclude from sitemap by mysql

I’m not entirely sure if that’s possible, but writing you a custom SQL query would go beyond the support that we offer in this forum. Why not use the designated field for this purpose in the XML Sitemap module?

Ok, but my question is: when I set in a post to exclude it from sitemap, what happens? There is a custom Field or whathever else?

If you check the “Exclude from sitemap” option, then that specific post/page is excluded from your sitemap. If you mean to ask whether there is a custom field to exclude pages, then yes – https://semperplugins.com/documentation/xml-sitemaps-module/#excluded-items