How to add a COUNT column as a subquery in MySQL

Hello I have 2 tables:

• purchase_info
• purchase_items

There's a view in which I display a summary of the purchase and I would like to count how many items belong to certain purchase.

How do I achieve this in a single query?

I would like to create an extra column like total_items, please note that Im using INNER JOINs to retrieve data from the clients and items tables, no problem with that.

count number of purchase_ids in table purchase_items and then join

So, query as follows (Using GROUP BY):

SELECT 
    purchase_info.*,
    T0.TOTAL_ITEMS
FROM purchase_info
LEFT JOIN (
    select purchase_id, 
        COUNT(purchase_id) AS TOTAL_ITEMS
    from purchase_items 
    GROUP BY purchase_id
) T0
ON purchase_info.id = T0.purchase_id
ORDER BY purchase_info.id DESC

Alternative query (using PARTITION BY):

SELECT 
    purchase_info.*,
    T0.TOTAL_ITEMS
FROM purchase_info
LEFT JOIN (
    select DISTINCT
        purchase_id, 
        COUNT(*) OVER (PARTITION BY purchase_id) AS TOTAL_ITEMS
    from purchase_items 
) T0
ON purchase_info.id = T0.purchase_id
ORDER BY purchase_info.id DESC

Don't use INNER JOIN. If some purchase_info data not available in purchase_items then it will not capture in INNER JOIN.
So, in this condition must use LEFT JOIN.