Help with custom SQL queries $wpdb->get_results()

I am trying to write some custom SQL queries into my WordPress templates, and then immediately iterate through the records and display them on the page (e.g. like a list of categories).

I came across the ‘$wpdb->get_results()’ syntax, which works perfectly fine for a simply query, e.g:

$wpdb->get_results("SELECT * FROM wp_posts WHERE field = 21");

However what I need to do next is to pass a value into the SQL query. Is this possible with the ‘get_results’ function?

I was hoping to do this as follows, but this does not work and I assume is not supported in the ‘get_results’ function:

$wpdb->get_results("SELECT * FROM wp_posts WHERE field= %d", 21)

Is this possible? And if not, is there an alternative which will work? Cheers.