Need help with nested queries

query
#1

I need help with a nested query and I can’t quite wrap my head around the correct syntax for what I am trying to do.

Essentially I am just trying to get the percentage of a certain tag (Item_Num) - I want to know the percentage of how many items in a certain transaction are of a particular product.

So if my particular item query was -

select count("Item") from "Transactions" where "Shopper" = $shopper AND "item_Num" = 'x16214' Group By "Transaction"

I would like to nest it in this query -

SELECT 
    $query / count("Items"),   <---- Here
	sum("Amount")/sum("Items"), 
	sum("Navigation")
FROM 
	"Transactions"
WHERE 
 	"Shopper" = $shopper
GROUP BY 
	"Transaction"

or like

SELECT 
    (select count("Item") from "Transactions" where "Shopper" = $shopper AND "item_Num" = ''x16214' Group By "Transaction") / count("Items"),
	sum("Amount")/sum("Items"), 
	sum("Navigation")
FROM 
	"Transactions"
WHERE 
 	"Shopper" = $shopper
GROUP BY 
	"Transaction"

Any help would be greatly appreciated
thank you