Question: Show all template type codes that are not used by any document.
Schema: create table documents (template_id varchar, PRIMARY KEY (template_id)); create table templates (template_id varchar, PRIMARY KEY (template_id)); create table templates (template_type_code varchar, PRIMARY KEY (template_type_code))
SQL: select template_type_code from templates except select template_type_code from templates as t1 join documents as t2 on t1.template_id = t2.template_id