Select values from second table that are not linked to first table

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Select values from second table that are not linked to first table

jmadsen 98862

SSC Eights!

Points: 914
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January 23, 2023 at 9:22 pm
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#4140972
This is a simplified version of the actual tables and query I'm working with.
I feel like it shouldn't be that difficult, maybe i'm not seeing the forest for the trees.
I have a product table with 3 products in it.
I have a product_user table with "links" to the products. An entry for each user and product they are a user for.
(poor design, but a) this is a simplified version of the actual design and b) I'm stuck with this design)
I need every product_user and the products they are NOT linked as a user for.
If Object_ID('TempDB..#product_user', 'U') is not null
drop table #product_user
Create table #product_user(id int, username varchar(50), prod_id int)
If Object_ID('TempDB..#product', 'U') is not null
drop table #product
Create table #product(id int, product_name varchar(50))
Insert into #product (id, product_name)
Select 1, 'abc' UNION ALL
Select 2, 'xyz' UNION ALL
Select 3, 'def'
Insert into #product_user (id, username , product_id)
Select 1, 'jdoe', 1 UNION ALL
Select 2, 'asmith', 1 UNION ALL
Select 3, 'asmith', 3
I would like to select every product_user and the product they are NOT linked to.
Desired result -
product_user, product_id
jdoe, 2
jdoe, 3
asmith, 2
- This topic was modified 1 year, 3 months ago by jmadsen 98862.
pietlinden

SSC Guru

Points: 63428
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January 23, 2023 at 10:06 pm
Answer
#4140984
Sounds like you need to cross join the two tables (creates all possible combinations... be careful with these!), and then subtract out the matches. (It would help us a LOT if you would put code tags around your code... just makes it easier to read. Thanks for the sample data - that's a huge help.
The CROSS JOIN part creates a unique list of all possible combinations of (ProductIID, UserID), and then you basically subtract out all the values in the #product_user table by using EXCEPT, and what's left is your answer.
Ideally, you would have 3 tables, not 2. One for Users, one for Products, and one for "uses". Then it's just
(Users X Products) - Uses
```
use tempdb;
go

CREATE TABLE #product_user(
id int,
username varchar(50),
prod_id int);

CREATE TABLE #product(id int, product_name varchar(50));
GO

Insert into #product (id, product_name)
VALUES (1, 'abc'),(2, 'xyz'),(3, 'def');

Insert into #product_user (id, username , prod_id)
VALUES (1, 'jdoe', 1),
(2, 'asmith', 1),
(3, 'asmith', 3);

/* select every product_user and the product they are not using */-- returns 9 records
SELECT xj.*
FROM
(SELECT ProductID = id, u.UserID
FROM #Product 
CROSS JOIN
(SELECT DISTINCT UserID = id
FROM #product_user) 
u) xj
EXCEPT 
(SELECT p_u.prod_id, p_u.id
FROM #product_user p_u);
```
- This reply was modified 1 year, 3 months ago by pietlinden.
- This reply was modified 1 year, 3 months ago by pietlinden.
January 24, 2023 at 7:22 am

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jmadsen 98862 SSC Eights! Points: 914 More actions · Answer 1

Thank you. This is what I needed. A cross join with an except.

I just modified for the username rather then the id and it returns exactly what I was looking for.

Applied it to my larger real life scenario and it works.

aitorzur@hotmail.com SSC Rookie Points: 32 More actions · Answer 2

I modified "pietlinden" solution to use a left join rather than EXCEPT. I find it more clear.

select xj.UserId, xj.ProductId
from (
select u.UserId,  ProductId = p.id
from ( select distinct UserId = id from #product_user ) u
cross join  #product p
) xj
left outer join #product_user pu
on xj.UserId = pu.Id
and xj.ProductId = pu.prod_id
where pu.Id is null

UserId      ProductId
----------- -----------
1           2
1           3
2           2
2           3
3           1
3           2

(6 rows affected)

aitorzur@hotmail.com SSC Rookie Points: 32 More actions · Answer 3

I modified "pietlinden" solution to use a left join rather than EXCEPT. I find it more clear.

select xj.UserId, xj.ProductId
from (
select u.UserId,  ProductId = p.id
from ( select distinct UserId = id from #product_user ) u
cross join  #product p
) xj
left outer join #product_user pu
on xj.UserId = pu.Id
and xj.ProductId = pu.prod_id
where pu.Id is null

UserId      ProductId
----------- -----------
1           2
1           3
2           2
2           3
3           1
3           2

(6 rows affected)

Jeff Moden SSC Guru Points: 1003863 More actions · Answer 4

Never mind... post deleted because it wasn't necessary. The OP figured it out.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Jeff Moden SSC Guru Points: 1003863 More actions · Answer 5

As a bit of a sidebar, this uses 31% fewer reads, which will come in handy for when the cross joined tables are larger...

 WITH 
 cteProduct AS (SELECT ProductID = ID FROM #Product)
,cteUser    AS (SELECT DISTINCT UserName = username FROM #product_user)
 SELECT u.UserName,p.ProductID
   FROM cteProduct p CROSS JOIN cteUser u
 EXCEPT
 SELECT username,prod_id
   FROM #product_user
  ORDER BY UserName, ProductID
;

Results:

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)