Combining two resulted columns on order by when one is null - mysql

Question

I have a inventory table where i have records which refers to inventory movement of products sold by a company. The movement have 'INCOMING', 'OUTGOING' params based on type of inventory movement. It also has one more column which says the type of 'INCOMING' or 'OUTGOING'... like incoming because of NEW STOCK arrival, outgoing because of PURCHASE by customer... etc...
Now am making a report where i want to list non sold products for a long while. So am making the following query...
SELECT p.id as pid, product_name, DATEDIFF(NOW(), MAX(case when movement_type='OUTGOING' and movement_type_category='PURCHASED' then movement_on end)) AS unsold_days_since_last_sale, DATEDIFF(NOW(), MIN(case when movement_type='INCOMING' and movement_type_category='NEW_STOCK' and quantity>0 then movement_on end)) AS unsold_days_since_first_inventory_in, MAX(case when movement_type='INCOMING' and movement_type_category='NEW_STOCK' and quantity>0 then movement_on end) AS last_inv_in from inventory_movement im left join products p on im.product = p.id GROUP BY product having last_inv_in > 0 ORDER BY unsold_days_since_last_sale desc limit 100
And i get the following output as shown in the image.
This output is nearly correct but with one issue. If a product was never sold even once in the past the column where i try to get days different between CURRENT DAY and LAST SOLD DAY will return null. In that case i need the DAYS difference between CURRENT DAY and FIRST INVENTORY IN of that product to be on the place so i can order that column descending and get the output. But i can get those data as only 2 different columns not as one column. Can someone help me to write a query to get it as combined column so i can sort that data to get result. Am attaching my inventory movement table snap also to show how the data look like...

Answer 1

I think IfNull function will resolve your issue.
Here's modified query.
SELECT p.id AS pid,
product_name,
Datediff(Now(), Ifnull(Max(CASE
WHEN movement_type = 'OUTGOING'
AND movement_type_category =
'PURCHASED' THEN
movement_on
end), Min(CASE
WHEN movement_type = 'INCOMING'
AND movement_type_category =
'NEW_STOCK'
AND quantity > 0 THEN
movement_on
end))) AS
unsold_days_since_last_sale,
Datediff(Now(), Min(CASE
WHEN movement_type = 'INCOMING'
AND movement_type_category = 'NEW_STOCK'
AND quantity > 0 THEN movement_on
end)) AS
unsold_days_since_first_inventory_in,
Max(CASE
WHEN movement_type = 'INCOMING'
AND movement_type_category = 'NEW_STOCK'
AND quantity > 0 THEN movement_on
end) AS last_inv_in
FROM inventory_movement im
LEFT JOIN products p
ON im.product = p.id
GROUP BY product
HAVING last_inv_in > 0
ORDER BY unsold_days_since_last_sale DESC
LIMIT 100