Select if distinct count > 2? - sql

Question

[EDITED 10/03/2019 as per request]
Have a large dataset and need to map 2 ids, on different rows, to one 'conflict id'.
For example.
These rows must be attributed to a single value, "Apples & Pears".
I then need to know which persons have both "Apple" and "Pears", through this value, not just one of then.
It's a many to one relationship, that must be complete. Users must not be attributed "Apples & Pears“ if they do not have BOTH requirements.
I have master table of one to one relation for each fruit to each combination, but only want them to be joined with user table if the user has both fruits.
Table 1: User, Fruit
Josh, Apple
Josh, Pear
Tom, Apple
Kate, Pear
Table 2: Fruit, Product
Apple, Apples&Pears
Pear, Apples&Pears
Table 3 (OUTCOME I WANT TO ACHIEVE):
Josh, Apple, Apples&Pears
Josh, Pear, Apples&Pears
Tom, Apple, NULL
Kate, Pear, NULL

Answer 1

Welcome to S/O. Typically you want to provide more realistic sample table structures and data to portray at least critical to your question. if ever private data, make up, but keep it in accurate context.
That being said, I don't think you actually have a table of apples, pears and apples & pears, but I will play along.
I assume (example) you have a table of people, and another table of things they are associated with in a child table. I would do a join per person FIRST, to those that DO have an association with the "apples & pears" record, then do a LEFT-JOIN to same child table once for apples, another for pears. You can determine what you want to do with results (such as delete from the individual apples, pears child records).
Select
p.LastName,
p.FirstName,
case when justApples.PersonID IS NULL
then 'No' else 'Yes' end as AlsoHasApples,
case when justPears.PersonID IS NULL
then 'No' else 'Yes' end as AlsoHasPears
from
Person p
JOIN Fruits f
on p.PersonID = f.personID
AND f.Description = 'Apples & Pears'
LEFT JOIN Fruits justApples
on p.PersonID = justApples.PersonID
AND f.Description = 'Apples'
LEFT JOIN Fruits justPears
on p.PersonID = justPears.PersonID
AND f.Description = 'Pears'
Since the first "(inner) JOIN" explicitly is looking for those that ARE marked as both apples & pears record, that minimizes the list to just those people pre-qualified.
THEN, the two "LEFT-JOIN" are explicitly looking for the OTHER individualized "Apples" and "Pears" respectively.
By testing the IS NULL will indicate if the person does (or not via NULL) have that item.
If you ONLY WANT to return those that are Apples & Pears AND have one or both of the other individual items, you can add a WHERE clause at the end such as
WHERE
justApples.PersonID IS NOT NULL
OR justPears.PersonID IS NOT NULL
An inverse would be to pre-qualify the justApples and justPears and NOT marked as associated with Apples & Pears such as by REQUIRING the JOINS to apples and pears individually and LEFT-JOIN the combination record
Select
p.LastName,
p.FirstName,
case when f.PersonID IS NULL
then 'No' else 'Yes' end as IsAssociatedWithApplesAndPearsRecord
from
Person p
JOIN Fruits justApples
on p.PersonID = justApples.PersonID
AND f.Description = 'Apples'
JOIN Fruits justPears
on p.PersonID = justPears.PersonID
AND f.Description = 'Pears'
LEFT JOIN Fruits f
on p.PersonID = f.personID
AND f.Description = 'Apples & Pears'
The two joins to the individual REQUIRE each individual part, and the LEFT-JOIN shows IF they are also marked as the combination entry. As you can see by the ambiguity in your question, providing a little more detail in what you want can significantly help getting a more accurate answer to your true needs. Try not to mask details to context, but mask the sample data in itself where more private issues are of concern.