Deleting duplicate rows, no unique keys - relational table - mysql

Question

I have a relational table that connects two other tables based on their IDs. There can be duplicates for both columns - but there CANNOT be the same row twice. I handle the checking code side.
How do I remove duplicate rows (see below):
select * from people:
a | b
1 2
1 3
1 3
1 7
2 3
2 5
2 5
2 9
I want the result to be:
a | b
1 2
1 3
1 7
2 3
2 5
2 9

Answer 1

This should work:
ALTER IGNORE TABLE people ADD UNIQUE (a,b);
If you don't want to add an index, then this should work:
DROP TABLE IF EXISTS people_old;
DROP TABLE IF EXISTS people_new;
CREATE TABLE people_new LIKE people;
INSERT INTO people_new SELECT DISTINCT * FROM people;
RENAME TABLE people TO people_old, people_new TO people;

Answer 2

There can be duplicates for both columns - but there CANNOT be the same row twice
That's a constraint on the table that you have not implemented. The constraint is a unique index on (a,b). If you had the index you would not have duplicates.
IMHO your best approach is to add the unique index to the table, using a temporary table to first remove the duplicates:
Copy person to person_temp
Delete all from person
Add unique index to person
Copy unique a,b from person_temp to `person.

Answer 3

This is how you can delete duplicate rows... I'll write you my example and you'll need to apply to your code. I have Actors table with ID and I want to delete the rows with repeated first_name
mysql> select actor_id, first_name from actor_2;
+----------+-------------+
| actor_id | first_name |
+----------+-------------+
| 1 | PENELOPE |
| 2 | NICK |
| 3 | ED |
....
| 199 | JULIA |
| 200 | THORA |
+----------+-------------+
200 rows in set (0.00 sec)
-Now I use a Variable called #a to get the ID if the next row have the same first_name(repeated, null if it's not).
mysql> select if(first_name=#a,actor_id,null) as first_names,#a:=first_name from actor_2 order by first_name;
+---------------+----------------+
| first_names | #a:=first_name |
+---------------+----------------+
| NULL | ADAM |
| 71 | ADAM |
| NULL | AL |
| NULL | ALAN |
| NULL | ALBERT |
| 125 | ALBERT |
| NULL | ALEC |
| NULL | ANGELA |
| 144 | ANGELA |
...
| NULL | WILL |
| NULL | WILLIAM |
| NULL | WOODY |
| 28 | WOODY |
| NULL | ZERO |
+---------------+----------------+
200 rows in set (0.00 sec)
-Now we can get only duplicates ID:
mysql> select first_names from (select if(first_name=#a,actor_id,null) as first_names,#a:=first_name from actor_2 order by first_name) as t1;
+-------------+
| first_names |
+-------------+
| NULL |
| 71 |
| NULL |
...
| 28 |
| NULL |
+-------------+
200 rows in set (0.00 sec)
-the Final Step, Lets DELETE!
mysql> delete from actor_2 where actor_id in (select first_names from (select if(first_name=#a,actor_id,null) as first_names,#a:=first_name from actor_2 order by first_name) as t1);
Query OK, 72 rows affected (0.01 sec)
-Now lets check our table:
mysql> select count(*) from actor_2 group by first_name;
+----------+
| count(*) |
+----------+
| 1 |
| 1 |
| 1 |
...
| 1 |
+----------+
128 rows in set (0.00 sec)
it works, if you have any question write me back