MySQL: Merge entries in pivot table based on joined table duplicate entries - mysql

Question

I have 2 tables, one of participants:
+----+------------+-----------+
| id | First Name | Last Name |
+----+------------+-----------+
| 0 | John | Snow |
| 1 | John | Snow |
| 2 | Michael | Jackson |
+----+------------+-----------+
And one pivot table which connects participants with events:
+----+----------------+----------+
| id | participant_id | event_id |
+----+----------------+----------+
| 0 | 0 | 12 |
| 1 | 1 | 35 |
| 2 | 2 | 35 |
+----+----------------+----------+
By mistake there are duplicate entries in the participants' table.
How I can delete duplicate entries in participants' table and update accordingly the pivot table? So the expected results will be :
participants:
+----+------------+-----------+
| id | First Name | Last Name |
+----+------------+-----------+
| 0 | John | Snow |
| | | | //deleted
| 2 | Michael | Jackson |
+----+------------+-----------+
pivot table:
+----+----------------+----------+
| id | participant_id | event_id |
+----+----------------+----------+
| 0 | 0 | 12 |
| 1 | 0 | 35 | //participant_id changed from 1 to 0
| 2 | 2 | 35 |
+----+----------------+----------+

Answer 1

This will be a multi-step process:
First step is to update the mapping table pivot. Following query will give you all the names which are Duplicate, and the first id for them:
SELECT first_name, last_name, MIN(id) AS first_id
FROM participants
GROUP BY first_name, last_name
HAVING COUNT(*) > 1 -- more than one rows means duplicates exist
You can use the above query as a subquery to update the pivot table using a series of Joins:
UPDATE pivot AS m
JOIN participants AS p1
ON p1.id = m.participant_id
JOIN (
SELECT first_name, last_name, MIN(id) AS first_id
FROM participants
GROUP BY first_name, last_name
HAVING COUNT(*) > 1
) AS p2 ON p2.first_name = p1.first_name
AND p2.last_name = p1.last_name
AND p2.first_id <> p1.id -- avoid the original row
SET m.participant_id = p2.first_id -- update the duplicate row's id to first id
Now, you can DELETE the duplicate rows using the same subquery (to find duplicates):
DELETE p1 FROM participants AS p1
JOIN (
SELECT first_name, last_name, MIN(id) AS first_id
FROM participants
GROUP BY first_name, last_name
HAVING COUNT(*) > 1
) AS p2 ON p2.first_name = p1.first_name
AND p2.last_name = p1.last_name
AND p2.first_id <> p1.id -- avoid the original row
Finally, fix the problem at your data definition level, to avoid this happening again, by defining a UNIQUE constraint on the (first_name, last_name)
ALTER TABLE participants ADD CONSTRAINT unq_idx_name UNIQUE(first_name, last_name);