Find duplicates per parent id - mysql

Question

I have two DB tabels which form a parent child relationship from Planung to Aufgabe:
Planung:
CREATE TABLE `planung` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`Bezeichnung` varchar(255) DEFAULT NULL,
-- lots of ohter columns
PRIMARY KEY (`id`),
) ENGINE=InnoDB
Aufgabe:
CREATE TABLE `aufgabe` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`planung_id` bigint(20) DEFAULT NULL, -- foreign key to Planung.id
`Nummer` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
-- lots of ohter columns
) ENGINE=InnoDB
I'm looking for a query which gives me all Planung.id and Aufgabe.ID for all duplicate Nummer per Planung. Or in other words: For every Planung Aufgabe.Nummer must be unique, I want to check whehter this is really the case in my DB (I know it isn't).

Answer 1

SELECT planung_id, GROUP_CONCAT(id) AS aufgabe_id, Nummer, COUNT(1) as num_duplicates
FROM aufgabe
GROUP BY planung_id, Nummer
HAVING COUNT(1) > 1
This query gives all planung_id which has duplicate Nummer and displays them like:
planung_id aufgabe_id Nummer num_duplicates
1 2,5,8 1 3
Which means, for Planung 1, there exists three Aufgabe with Nummer 1, and they are 2,5 and 8.
Edit: Shamelessly stolen from #dgw's comment:
Once you have run this query, and corrected all the duplicates. Add a unique index to aufgabe {planung_id, Nummer} to ensure that the database maintains this constraint:
ALTER TABLE aufgabe
ADD CONSTRAINT UNIQUE uq_planung_id_and_nummer (planung_id, Nummer)

Answer 2

select p.id, a.id, a.Nummer from planung p
inner join aufgabe a on a.planung_id = p.id
group by p.id, a.id, a.Nummer having count(*) > 1

Answer 3

This will give you all elements from planung being more than once a parent.
SELECT planung_id,nummer,COUNT(*)
FROM aufgabe
GROUP BY 1,2 HAVING COUNT(*)>1 ;

Answer 4

This will give you every planung_id that has multiple Nummer values:
SELECT planung_id
FROM aufgabe
GROUP BY planung_id
HAVING COUNT(DISTINCT Nummer) > 1
If you also need the aufgabe.id, you can do this:
SELECT *
FROM aufgabe
WHERE
planung_id IN (
<query above>
)
If you need to enforce that at all times, consider adding a key on {planung_id, Nummer} (as #dgw has already suggested).