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Delete duplicated mysql rows with no primary key
user_id category_id
1            2
1            3
1            4
1            2
2            2
2            3
2            2

CREATE TABLE temp SELECT DISTINCT * FROM tablename;
 ALTER TABLE tablename RENAME junk;
 ALTER TABLE temp RENAME tablename;

insert into new_better_table
select user_id, category_id from old_table group by user_id, category_id