仅选择非唯一的行[MySQL]

Question

I'm trying to make a query that will check for duplicates, and if I find duplicates I want to be able to update those accordingly. For example, I have the following data set, how do I only select the 2 rows that have the same value for 'position'?

 ID position 1 0 2 1 3 2 4 1 5 3 

In this example I would select rows 2 and 4 only.

Answer 1

SELECT t.position, 
    count(t.*) AS count 
FROM table t
GROUP BY position 
HAVING count(position) > 1

Should return an a list of positions and their repetition counts.

Edit: Neglected to alias my table

If you want to see the rows that have duplicate positions try:

SELECT t1.ID,
    t1.position,
    t2.ID,
    t2.position
FROM table t1
INNER JOIN table t2
    ON t1.ID < t2.ID
    AND t1.position = t2.position

This will give you a set of rows (4 columns) where there are duplicates. You will get 1 row for a duplicate, 3 for a triplicate, 6 for a quad, and it increases from that point on.

Answer 2

To expand a little on Matt S's answer (which is right on), you could use his results and join back to the original table to get the exact rows:

select
    id
    ,position
from table t
    join (
        select t1.position
        from table t1
        group by t1.position
        having count(*) > 1
    ) list
    on t.position = list.position

Using a CTE or an EXISTS() function in a WHERE clause would make this a little easier to read, but I don't know what MySQL syntactically supports (I'm a T-SQL guy :) ). This should be ANSI.

Answer 3

Matt S's answer is the way you would generally get what you're looking for. To expand slightly on scottE's answer (and since we're talking MySQL), you could also use group_concat() to pull together the info you're looking for, if the output is pleasing.

SELECT group_concat(ID) as 'IDs', position
FROM table
GROUP BY position
HAVING count(position) > 1

should return you:

IDs | position
2, 4 | 1