LunaQL docs are incomplete.
Reference
clauses
select

select

The select clause is used to select specific properties:

users.lunaql
query({
	from({
		users: {
			select([
				"first_name",
				"last_name",
				"email_address"
			]);
		};
	});
});

You may also merge 2 or more properties in your select clause using the mutate short function:

users.lunaql
query({
	from({
		users: {
			select([
				"mutate => $.first_name $.last_name => full_name",
				"email_address"
			]);
		};
	});
});