Rewrite a where in / intersect query to a join - sql

Question

Is there a way to rewrite my query into a join one.
The question I have to solve is: List the names of green items sold by no department on the first floor. Do not show duplicates.
select distinct itemname from xsale where deptname in (
select deptname from xdept where deptfloor <> 1
)
intersect (
select itemname from xitem where itemcolor= 'green'
)
I have been stuck at this exercise a couple of days now because the join statements don't make much sense to me even after reading about it. I hope someone can help me.

Answer 1

I think the following query will help you about how to use join statment for your query;
EDITED
select distinct xs.itemname from xsale xs
inner join xdept xp on xs.deptname=xp.deptname
where xp.deptfloor <>1
intersect
(select xi.itemname from xitem xi where xi.itemcolor= 'green')

Answer 2

Check if this works for you
Select Distinct a.itemname from xsale a
INNER JOIN xdept b on a.deptName = b.deptName
INNER JOIN xitem c on a.itemName = c.itemname
Where b.deptfloor <> 1 and c.itemcolor = 'green'

Answer 3

I think you can express the logic as an EXIST and NOT EXIST query:
SELECT itemname
FROM xitem
WHERE itemcolor = 'green' -- all green items
AND EXISTS (
-- exists a sale for that item
SELECT 1
FROM xsale
WHERE xsale.itemname = xitem.itemname
AND NOT EXISTS (
-- not exists a department in those sales with floor = 1
SELECT 1
FROM xdept
WHERE xdept.deptname = xsale.deptname AND xdept.deptfloor = 1
)
)

Answer 4

It is often best to build your queries up step by step. So if we break the problem down:
List the names of green items sold
SELECT i.ItemName
FROM xitem AS i
WHERE i.ItemColor = 'Green';
Items sold by a department on the first floor
SELECT s.ItemName
FROM xsale AS s
INNER JOIN xdept AS d
ON d.DeptName = s.DeptName
WHERE d.DeptFloor = 1;
So now, you want all the items output by the first query, except for those that appear in the 2nd:
SELECT i.ItemName
FROM xitem AS i
WHERE i.ItemColor = 'Green'
EXCEPT
SELECT s.ItemName
FROM xsale AS s
INNER JOIN xdept AS d
ON d.DeptName = s.DeptName
WHERE d.DeptFloor = 1;
Then the final part:
Do not show duplicates:
SELECT DISTINCT i.ItemName
FROM xitem AS i
WHERE i.ItemColor = 'Green'
EXCEPT
SELECT s.ItemName
FROM xsale AS s
INNER JOIN xdept AS d
ON d.DeptName = s.DeptName
WHERE d.DeptFloor = 1;
An alertanative to EXCEPT would be NOT EXISTS, they will almost always result in the same execution plan, but I find NOT EXISTS is more flexible (you don't need the same columns in both queries):
SELECT i.ItemName
FROM xitem AS i
WHERE i.ItemColor = 'Green'
AND NOT EXISTS
( SELECT 1
FROM xsale AS s
INNER JOIN xdept AS d
ON d.DeptName = s.DeptName
WHERE d.DeptFloor = 1
AND s.ItemName = i.ItemName
)
GROUP BY i.ItemName;
Again to show an alternative, I have used GROUP BY rather than DISTINCT. In most cases these are semantically equivalent, but there are scenarios where GROUP BY will perform better (namely when a scalar function is involved - GROUP BY will remove duplciates first, and then execute the funciton on all remaining values, DISTINCT will execute the function first and remove duplicate results).
Examples on DB Fiddle