SQL Query - Check for Two Distinct Values - sql

Question

Given the below data set I want to run a query to highlight any 'pairs' that do not consist of a 'left' and 'right'.
+---------+-----------+---------------+----------------------+
| Pair_Id | Pair_Name | Individual_Id | Individual_Direction |
+---------+-----------+---------------+----------------------+
| 1 | A | A1 | Left |
| 1 | A | A2 | Right |
| 2 | B | B1 | Right |
| 2 | B | B2 | Left |
| 3 | C | C1 | Left |
| 3 | C | C2 | Left |
| 4 | D | D1 | Right |
| 4 | D | D2 | Left |
| 5 | E | E1 | Left |
| 5 | E | E2 | Right |
+---------+-----------+---------------+----------------------+
In this instance Pair 3 'C' has two lefts. Therefore, I would look to display the following:
+---------+-----------+---------------+----------------------+
| Pair_Id | Pair_Name | Individual_Id | Individual_Direction |
+---------+-----------+---------------+----------------------+
| 3 | C | C1 | Left |
| 3 | C | C2 | Left |
+---------+-----------+---------------+----------------------+

Answer 1

You can simply use not exists:
select t.*
from t
where not exists (select 1
from t t2
where t2.pair_id = t.pair_id and
t2.Individual_Direction <> t.Individual_Direction
) ;
With an index on (pair_id, Individual_Direction), this should not only be the most concise solution but also the fastest.
If you want to be sure that there are pairs (the above returns singletons):
select t.*
from t
where not exists (select 1
from t t2
where t2.pair_id = t.pair_id and
t2.Individual_Direction <> t.Individual_Direction
) and
exists (select 1
from t t2
where t2.pair_id = t.pair_id and
t2.Individual_ID <> t.Individual_ID
);
You can also do this using window functions:
select t.*
from (select t.*,
count(*) over (partition by pair_id) as cnt,
min(status) over (partition by pair_id) as min_status,
max(status) over (partition by pair_id) as max_status
from t
) t
where cnt > 1 and min_status <> max_status;

Answer 2

One option uses aggregation:
WITH cte AS (
SELECT Pair_Name
FROM yourTable
WHERE Individual_Direction IN ('Left', 'Right')
GROUP BY Pair_Name
HAVING MIN(Individual_Direction) = MAX(Individual_Direction)
)
SELECT *
FROM yourTable
WHERE Pair_Name IN (SELECT Pair_Name FROM cte);
The HAVING clause used above asserts that a matching pair has both a minimum and maximum direction which are the same. This implies that such a pair only has one direction.
As is the case with Gordon's answer, an index on (Pair_Name, Individual_Direction) might help performance:
CREATE INDEX idx ON yourTable (Pair_Name, Individual_Direction);

Answer 3

There should be an elegant way of using window function than what I wrote:
WITH ranked AS
(
SELECT *, RANK() OVER(ORDER BY Pair_Id, Pair_Name, Individual_Direction) AS r
FROM pairs
),
counted AS
(
SELECT Pair_Id, Pair_Name, Individual_Direction,r, COUNT(r) as times FROM ranked
GROUP BY Pair_Id, Pair_Name, Individual_Direction, r
HAVING COUNT(r) > 1
)
SELECT ranked.Pair_Id, ranked.Pair_Name, ranked.Individual_Id, ranked.Individual_Direction FROM ranked
RIGHT JOIN counted
ON ranked.Pair_Id=counted.Pair_Id
AND ranked.Pair_Name=counted.Pair_Name
AND ranked.Individual_Direction=counted.Individual_Direction