sql query on a custom controller

Hi

I need to run a query on a WP table and return the results. I count find any examples on this.

I need to know if I could use the global $wpdb value and run a sql command like in a normal WP page or if there is another way to do this.

I tried the $wpdb way and it doesn’t seem to work. Struggling with this for days.

thanks in advance

https://www.ads-software.com/plugins/json-api/