Number duplicate records on the MySQL table - mysql

Question

Have a table with similar schema
id control code amount
1 200 12 300
2 400 12 300
3 200 12 300
4 100 10 400
5 100 10 400
6 500 13 500
Trying to list the duplicates of records on a UI.
Using following query I can retrieve the duplicate records and show it on UI.
select * from mwt group by control,code,amount having count(id) > 1;
id control code amount
1 200 12 300
4 100 10 400
Here the records with id 1 and 4 are duplicates of 3 and 5 respectively.
On the UI, the user will click a check-box adjacent to the record and corresponding duplicate records should be populate to the UI. To make things easier trying to populate another column named dup_id. Using this dup_id it is possible to filter the results from UI , which is in the JSON format.
How to create a result set similar to the one shown below?
id control code amount dup_id
1 200 12 300 1
2 400 12 300
3 200 12 300 1
4 100 10 400 4
5 100 10 400 4
6 500 13 500

Answer 1

This seems like a simpler solution than that suggested by #kickstarter - but maybe I've misunderstood the requirement...
SELECT x.*
, y.dup_id
FROM my_table x
LEFT
JOIN
( SELECT MIN(id) dup_id
, control
, code
, amount
FROM my_table
GROUP
BY control
, code
, amount
HAVING COUNT(*) > 1
) y
ON y.control = x.control
AND y.code = x.code
AND y.amount = x.amount;

Answer 2

Depending on how accurate the order has to be, you could do something like this.
This is getting all the unique control / code / amount with a count, to get a flag to know if that is a duplicate row, and ordered by control / code / amount so that they are in order. It does a cross join to initialise a few user variables.
Then it calculates a counter, only incrementing it if any of control / code / amount have changed AND it is a duplicate row. Then sets user variables to store the previous values of control / code / amount.
The outer query then orders the results back in to id order.
SELECT sub3.id,
sub3.control,
sub3.code,
sub3.amount,
sub3.dup_id
FROM
(
SELECT sub2.id,
sub2.control,
sub2.code,
sub2.amount,
#cnt:=IF(#control=control AND #code=code AND #amount=amount AND sub2.id_count IS NOT NULL, #cnt, IF(sub2.id_count IS NULL, #cnt, #cnt + 1)),
#control:=control,
#code:=code,
#amount:=amount,
IF(sub2.id_count IS NULL, NULL, #cnt) AS dup_id
FROM
(
SELECT mwt.id, mwt.control, mwt.code, mwt.amount, sub1.id_count
FROM mwt
LEFT OUTER JOIN
(
SELECT control, code, amount, COUNT(id) AS id_count
FROM mwt
GROUP BY control,code,amount
HAVING id_count > 1
) sub1
ON mwt.control = sub1.control
AND mwt.code = sub1.code
AND mwt.amount = sub1.amount
ORDER BY mwt.control, mwt.code, mwt.amount
) sub2
CROSS JOIN
(
SELECT #cnt:=0, #control:=0, #code:=0, #amount:=0
) sub0
) sub3
ORDER BY id
Note that this is ordering by control, code and amount, so not an exact match for your required output (which would require getting the first duplicates ordered by id first).
EDIT - Simpler and better way to do it. This gets all the duplicate rows with the min id for those duplicates (ordered by the min id), and uses a user variable to add a sequence number for those. Then LEFT OUTER JOINs that back against the main table to put that sequence number in all the matching rows.
SELECT mwt.id, mwt.control, mwt.code, mwt.amount, sub2.dup_id
FROM mwt
LEFT OUTER JOIN
(
SELECT sub1.id, sub1.control, sub1.code, sub1.amount, #cnt:=#cnt+1 AS dup_id
FROM
(
SELECT MIN(id) AS id, control, code, amount
FROM mwt
GROUP BY control,code,amount
HAVING COUNT(id) > 1
ORDER BY id
) sub1
CROSS JOIN
(
SELECT #cnt:=0
) sub0
) sub2
ON mwt.control = sub2.control
AND mwt.code = sub2.code
AND mwt.amount = sub2.amount
ORDER BY mwt.id

Answer 3

Would you need a dup_id column ?. I hope this can be achieved with a simple query like below
select id
, control
, code
, amount
from table
where control = from selected Record
and code = from selected Record
and amount = from selected Record
and id not equals from selected Record
You can very well omit the last not equals if the requirement is to list down duplicates including the selected record.