Sql query - Struggling to write/structure - sql

Question

I work for a distribution centre in the uk and i've recently started using sql queries to access data directly from the db.
I've been tasked with writing a query that gives the total number of locations we have in each aisle (from table locn_hdr) and total number of locations emtpy in each of those aisles. I'vr managed to get all the info i need but by using twp seperate queries. I'm struggling to combine them into the below headers
Aisle - count of locations - count of empty locations
The two queries i have are below
Count of locations
select AISLE, COUNT(AISLE)
from LOCN_HDR LH
where LH.LOCN_CLASS = 'A'
and BAY >= '0030'
AND BAY <= '0230'
AND PICK_DETRM_ZONE LIKE 'HG%'
AND LH.AISLE <= 'QA'
group by aisle
Order by aisle;
 
Count of empties
SELECT  aisle, COUNT(dsp_locn)
FROM locn_hdr lh
WHERE lh.locn_class = 'A'
AND bay >= '0030'
AND bay <= '0230'
AND pick_detrm_zone LIKE 'HG%'
AND lh.aisle <= 'QA'
AND NOT EXISTS
(SELECT 1
FROM wm_inventory wi
WHERE wi.location_id = lh.locn_id
AND wi.on_hand_qty > '0')
GROUP BY aisle
ORDER BY aisle;
Ideally indont just want the answer with the sql re written. I want to understand how i can do something similar myself in the future.
Thanks in advance guys! Sorry if I havent given enough info, go easy on me i'm new!
edit
Hi, thanks for the help first of all! It is appreciated. However, it isn't working as i need it. the column count(lh.aisle) is counting the number of empty locations rather than the total number of locations in the aisle. I had to change the sql slightly because i was getting error messages so i temporarily used
SELECT lh.AISLE, COUNT(lh.AISLE), COUNT(wi.location_id) -- count(lh.aisle) gives me the total empty locatins. Count(wi.location_id) gives me nothing...
FROM LOCN_HDR lh
LEFT OUTER JOIN wm_inventory wi ON wi.location_id = lh.locn_id AND wi.on_hand_qty > '0'
WHERE lh.LOCN_CLASS = 'A'
AND lh.BAY BETWEEN '0030' AND '0230'
AND lh.PICK_DETRM_ZONE LIKE 'HG%'
AND lh.AISLE <= 'QA'
AND wi.location_id IS NULL -- where there is no matching record for lh.locn_id with a quantity > 0
GROUP BY lh.AISLE
ORDER BY lh.AISLE;
This has given me a count of the empty locations which is fantastic. But I don't have a count of the total number of locations (empty or not).
Any more ideas would be appreciated!

Answer 1

SELECT lh.AISLE, COUNT(lh.AISLE), COUNT(wi.dsp_locn)
FROM LOCN_HDR lh
LEFT OUTER JOIN wm_inventory wi ON wi.location_id = lh.locn_id AND wi.on_hand_qty > '0'
WHERE lh.LOCN_CLASS = 'A'
AND lh.BAY BETWEEN '0030' AND '0230'
AND lh.PICK_DETRM_ZONE LIKE 'HG%'
AND lh.AISLE <= 'QA'
AND wi.ID IS NULL -- where there is no matching record for lh.locn_id with a quantity > 0
GROUP BY lh.AISLE
ORDER BY lh.AISLE;
Something similar to this should work, although I can't really test it without data. Your two queries are basically the same, with the notable exception of AND NOT EXISTS (... FROM wm_inventory ...).
Basically, you can LEFT OUTER JOIN this table on location_id and on_hand_qty. LEFT OUTER JOIN will return null where there is no match (instead of excluding them).
This means that if you have a record in LOCN_HDR with locn_id = 1, and you also have a record in wm_inventory with location_id = 1 but a quantity of 2, you will receive a NULL for this record.
Combined with the WHERE wi.ID IS NULL (change to suit the column on the wi table), this replaces the NOT EXISTS clause in the second query.
You may run into an issue with selecting COUNT(wi.dsp_locn) due to the GROUP BY, but you should be able to add GROUP BY lh.AISLE, wi.dsp_locn - this will combine all entries that have both the same lh.AISLE AND wi.dsp_locn.
If you need any additional explanation, please let me know - I'd be happy to give it a shot.