Merge references to duplicate rows in mysql - mysql

Question

This feels very simple and complex at the same time, but I can't quite get my head around an appropriate way of going about this as mysql query.
I have a table of tags called categories that should only have unique titles for the field cat_title. However, I've noticed that there are multiple rows with the same cat_title field name.
I want to delete all but the first instance of any duplicates. Simple enough, yes. But another table, tagging has a field called tagging_cat_id that references the identifier field, cat_id in the categories table. Deleting duplicates will break these references and point to nothing.
So, the more complex aspect is finding any tagging_cat_id field that references a duplicate row that's about to be deleted and change it to reference the (soon to be unique, single) first row of this cat_title
I am a novice at mysql and this is a bit out of my depth. I was almost tempted to do this manually by hand in a gui. Is there a simple enough method of doing this as a query that I could run on occasion to perform the above? (until what's causing duplicates to be created is resolved). Distrib version is 5.7.21.
Sample Data
Categories
+--------+-----------+
| cat_id | cat_title |
+--------+-----------+
| 1 | green |
| 2 | red |
| 3 | blue |
| 4 | green |
| 5 | green |
| 6 | red |
| 7 | white |
+--------+-----------+
Tagging
+------------+-------------------+----------------+
| tagging_id | tagging_record_id | tagging_cat_id |
+------------+-------------------+----------------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 7 |
| 4 | 3 | 5 |
| 5 | 4 | 6 |
| 6 | 5 | 4 |
| 7 | 5 | 3 |
| 8 | 6 | 5 |
+------------+-------------------+----------------+
I want to convert the above to the following:
Categories
+--------+-----------+
| cat_id | cat_title |
+--------+-----------+
| 1 | green |
| 2 | red |
| 3 | blue |
| 7 | white |
+--------+-----------+
Tagging
+------------+-------------------+----------------+
| tagging_id | tagging_record_id | tagging_cat_id |
+------------+-------------------+----------------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 7 |
| 4 | 3 | 1 |
| 5 | 4 | 2 |
| 6 | 5 | 1 |
| 7 | 5 | 3 |
| 8 | 6 | 1 |
+------------+-------------------+----------------+

Answer 1

If your version of MySql is 8.0+ you can use this query:
SELECT cat_id, MIN(cat_id) OVER (PARTITION BY cat_title) min_id
FROM categories
to identify for each cat_id the minimum cat_id with the same cat_title so you can update the table:
WITH ids AS (
SELECT cat_id, MIN(cat_id) OVER (PARTITION BY cat_title) min_id
FROM categories
)
UPDATE tagging t
INNER JOIN ids i ON i.cat_id = t.tagging_cat_id
SET t.tagging_cat_id = i.min_id
Then you can delete the duplicates:
WITH ids AS (
SELECT cat_id, MIN(cat_id) OVER (PARTITION BY cat_title) min_id
FROM categories
)
DELETE c
FROM categories c INNER JOIN ids i
ON i.cat_id = c.cat_id AND i.min_id < c.cat_id
See the demo.
For previous versions of MySql that do not support window functions and CTEs:
UPDATE tagging t
INNER JOIN categories c ON c.cat_id = t.tagging_cat_id
INNER JOIN (
SELECT cat_title, MIN(cat_id) min_id
FROM categories
GROUP BY cat_title
) m ON m.cat_title = c.cat_title
SET t.tagging_cat_id = m.min_id
and:
DELETE c1
FROM categories c1 INNER JOIN categories c2
ON c2.cat_title = c1.cat_title
WHERE c1.cat_id > c2.cat_id
See the demo.
Results:
cat_id
cat_title
1
green
2
red
3
blue
7
white
and:
tagging_id
tagging_record_id
tagging_cat_id
1
1
1
2
1
2
3
2
7
4
3
1
5
4
2
6
5
1
7
5
3
8
6
1