Mysql Duplicate Rows ( Duplicate detected using 2 columns )

How to remove duplicated in this setup?

id开发者_Go百科    A       B 
----------------
1     apple   2  
2     orange  1       
3     apple   2   
4     apple   1 

In here I want to remove (apple,2) which occurs twice. The id numbers are unique. I would use DISTINCT keyword if it were not. Can I some how make a key out of columns A and B and then use the DISTINCT keyword on that to get what I need ? Many thanks for your replies.

delete from myTable 
where id not in
(select min(id)
from myTable
group by A, B)

i.e. the select in brackets returns the first id for each grouping of A and B; deleting all ids that are not in this set will remove all occurences of an A-plus-B combination that are "subsequent" to its first occurrence.

EDIT: this syntax seems to be problematic: see bug report:

http://bugs.mysql.com/bug.php?id=5037

A possible workaround is to do this:

delete from myTable 
where id not in
(
      select minid from 
      (select min(id) as minid from myTable group by A, B) as newtable
) 

Yet another (from http://labs.creativecommons.org/2010/01/12/removing-duplicate-rows-in-mysql/). Add a unique index then delete it:

ALTER IGNORE TABLE mytable ADD UNIQUE INDEX tmpindex (A,B);
ALTER TABLE mytable DROP INDEX tmpindex;

The IGNORE keyword is a mysql extension that makes it drop rows that violate the UNIQUE keyword instead of just failing.

DELETE FROM fruit_table FT1
WHERE EXISTS
(
    SELECT * FROM fruit_table FT2 
    WHERE FT2.fruit_name_column = FT1.fruit_name_column
    AND   FT2.fruit_integer_column = FT1.fruit_integer_column
    AND   FT2.id <> FT1.id
)

This assumes you don't care which of the duplicate records is removed.

DELETE
FROM mytable
USING mytable, mytable AS vtable
WHERE vtable.id > mytable.id
AND mytable.A = vtable.A
AND mytable.A = vtable.A

You could use a temporary table with the data you want:

insert into temp_table
select min(id), A, B
 group by A, B

I'm not exactly sure what you're asking here. If you don't want duplicates of the A and B columns, then do just what you mentioned SELECT DISTINCT A, B FROM XXX. Maybe you could post an example of the type of result you would like to see.