SQL how to select unique one-to-one pairings - mysql

Question

edited to make clearer - many apologies for the confusion of the original example
I have the following table structure representing married couples:
id | Person | Spouse
______________________
1 | Mary | John
2 | John | Mary
3 | Katy | Bob
4 | Bob | Katy
5 | Mary | John
6 | John | Mary
In this example Mary is married to John, Katy to Bob and a different Mary is married to a different John.
How can I retrieve these pairs of married couples?
I have got close with this:
SELECT
p.id id1,
q.id id2
FROM
people p
INNER JOIN people q ON
p.person = q.spouse AND
q.person = p.spouse AND
p.id < q.id
ORDER BY p.id
However this returns:
1 | 2 (1st Mary & 1st John)
1 | 6 (1st Mary & 2nd John) *problem*
2 | 5 (1st John & 2nd Mary) *problem*
3 | 4 (Katy & Bob)
5 | 6 (2nd Mary & 2nd John)
How can I make sure the 1st Mary and 1st John are only married once (i.e. remove the problem rows above)?
Many thanks
Here's the SQL to create the example:
CREATE TABLE people
(`id` int, `person` varchar(7), `spouse` varchar(7))
;
INSERT INTO people
(`id`, `person`, `spouse`)
VALUES
(1, 'Mary', 'John'),
(2, 'John', 'Mary'),
(3, 'Katy', 'Bob'),
(4, 'Bob', 'Katy'),
(5, 'Mary', 'John'),
(6, 'John', 'Mary')
;
SELECT
p.id id1,
q.id id2
FROM
people p
INNER JOIN people q ON
p.person = q.spouse AND
q.person = p.spouse AND
p.id < q.id
ORDER BY p.id
;

Answer 1

I'll give it a try:
SELECT
p.id AS id1,
q.id AS id2
FROM
people AS p
JOIN people AS q ON
p.person = q.spouse AND
q.person = p.spouse AND
p.id < q.id
JOIN (SELECT
p.id, COUNT(*) AS rank
FROM
people AS p
INNER JOIN people AS p2 ON
p.person = p2.person AND
p.spouse = p2.spouse AND
p.id >= p2.id
GROUP BY p.id
) AS x ON
x.id = p.id
JOIN (SELECT
p.id, COUNT(*) AS rank
FROM
people AS p
INNER JOIN people AS p2 ON
p.person = p2.person AND
p.spouse = p2.spouse AND
p.id >= p2.id
GROUP BY p.id
) AS y ON
y.id = q.id AND
y.rank = x.rank ;
And another one:
SELECT
p.id AS id1,
q.id AS id2
FROM
people AS p
JOIN people AS q ON
p.person = q.spouse AND
q.person = p.spouse
JOIN people AS p2 ON
p.person = p2.person AND
p.spouse = p2.spouse AND
p.id >= p2.id
JOIN people AS q2 ON
q.person = q2.person AND
q.spouse = q2.spouse AND
q.id >= q2.id
WHERE
p.id < q.id
GROUP BY
p.id, q.id
HAVING
COUNT(DISTINCT p2.id) = COUNT(DISTINCT q2.id) ;
Both tested at SQL-Fiddle
It would be much simpler, if only MySQL had window functions (like almost all other DBMS have). Tested at Postgres fiddle:
WITH cte AS
( SELECT
id, person, spouse,
ROW_NUMBER() OVER( PARTITION BY person, spouse
ORDER BY id )
AS rn
FROM
people
)
SELECT
p.id AS id1,
q.id AS id2
FROM
cte AS p
JOIN cte AS q ON
p.person = q.spouse AND
q.person = p.spouse AND
p.rn = q.rn AND
p.id < q.id ;

Answer 2

In this example Mary is married to John, Katy to Bob and a different Mary is married to Richard.
Nothing in your show data structures allows to differentiate between those two “Marys”, because there is no difference between them.
Both are just the text literal Mary. If you want to differentiate between different people that might have the same name, then you need another criterion, and a unique one at that. (F.e. the id of the database records for each individual person.)

Answer 3

Your database stricture is wrong.
People like Mary, John, etc. do not have identity.
Some heuristic query might help, but it is not a reliable solution.
So, please, improve you data structure.

Answer 4

Not very elegant, but works:
SELECT p.id, q.id
FROM people p
INNER JOIN people q ON
p.person1 = q.person2 and
q.person1 = p.person1
which in fact uses the existance of an inverted row as a selector

Answer 5

There's lots of ways of doing it, however one of the most important reasons for using a database is that it holds lots of data - and there should rarely be times when you ever write a query which retrieves lots of data. Except in very unusual circumstances, and for homework assignments, the results should be filtered according to some criteria. Hence the most appropriate solution depends on what other stuff you add to the query later.
But here's a couple of examples of how to get the unique pairs:
SELECT a, b, GROUP_CONCAT(id)
(SELECT id
, IF (person>=spouse, person, spouse) as a
, IF (person>=spouse, spouse, person) as b
FROM yourtable ) AS pairs
GROUP BY a,b;
SELECT id, person, spouse
FROM yourtable s1
WHERE NOT EXISTS ( SELECT 1
FROM yourtable s2
WHERE s2.id>s1.id
AND s1.person=s2.spouse
AND s1.spouse=S2.person);
(there are several other solutions).