如何编写 SQL 查询以查找与另一个表中的记录相关的记录而忽略另一条记录

Question

I have a requirement to write a query that finds records related to a record in another table that aren't related to another record.

Below is an example of what I mean. I will happily rewrite this question and title if I can express the question in a better way (advice welcome).

Table company

id  
1
2
3

Table company_partner

id  company_id  company_name
1   1           Nike
2   1           Reebok
3   2           Nike
4   3           Nike

In the above example, I would like all companies partnered with Nike but not if they are also partnered with Rebook. Using the above example that would be companies 2 and 3 .

I can write a query that gives me all companies partnered with Nike:

SELECT c.id
FROM company c
INNER JOIN company_partner cp ON c.id = cp.company_id
WHERE 
    cp.company_name = 'Nike'
-- ignore cp.company_name = 'Reebok' ???

I am unclear how I can ignore companies that are also partnered with Reebok?

Answer 1

You should be able to use not in - like this:

SELECT c.id
FROM company c
INNER JOIN company_partner cp ON c.id = cp.company_id
WHERE  cp.company_name = 'Nike' 
AND c.id not in (
  select id from company_partner where company_name = 'Reebok'
)

Answer 2

Aggregation provides one straightforward option:

SELECT company_id
FROM company_partner
GROUP BY company_id
HAVING COUNT(CASE WHEN company_name = 'Nike' THEN 1 END) > 0 AND
       COUNT(CASE WHEN company_name = 'Reebok' THEN 1 END) = 0;