Left Outer Join returning results to be excluded - sql

Question

I have a query like
WITH a AS ( SELECT *
FROM inventory_tagalongs
WHERE TagAlong_ItemNum <> 'bokiwi2'
)
SELECT inventory.itemnum
, inventory.itemname
, inventory.ItemType,inventory.dept_id
FROM inventory
LEFT OUTER JOIN a
ON inventory.itemnum = a.itemnum
JOIN departments
ON inventory.dept_id = departments.dept_id
JOIN categories
ON departments.subtype = categories.cat_id AND categories.description = 'vapors'
in which i am trying to exclude a's results from the lower syntax but the results return are exactly the same whether or not left outer join..... is there or not.
Do i have my syntax incorrect here?

Answer 1

Good afternoon. Please try this:
select
inventory.itemnum
, inventory.itemname
, inventory.ItemType
, inventory.dept_id
from inventory
join departments on inventory.dept_id=departments.dept_id
join categories on departments.subtype=categories.cat_id and categories.description='vapors'
WHERE NOT EXISTS(
select 1 from inventory_tagalongs B where inventory.itemnum = b.itemnum
AND B.TagAlong_ItemNum <>'bokiwi2');
The CTE overthought the issue a bit. This accomplishes the goal with easier-to-read code and should perform better as well.
Thanks, John.

Answer 2

If you are trying to get results that don't have the tag, your logic is inverted. You want:
SELECT i.itemnum, i.itemname, i.ItemType, i.dept_id
FROM inventory i JOIN
departments d
ON i.dept_id = d.dept_id JOIN
categories c
ON d.subtype = c.cat_id AND
c.description = 'vapors' LEFT OUTER JOIN
a
ON i.itemnum = a.itemnum
WHERE a.itemnum IS NULL;
You don't need a CTE for this at all. It would usually be written as:
WITH a AS (
)
SELECT i.itemnum, i.itemname, i.ItemType, i.dept_id
FROM inventory i JOIN
departments d
ON i.dept_id = d.dept_id JOIN
categories c
ON d.subtype = c.cat_id AND
c.description = 'vapors' LEFT OUTER JOIN
inventory_tagalongs it
ON i.itemnum = it.itemnum AND it.TagAlong_ItemNum = 'bokiwi2'
WHERE a.itemnum IS NULL;
And -- as mentioned in another answer -- NOT EXISTS is another typical way to approach this problem.