Finding duplicate values in MySQL

Question

I have a table with a varchar column, and I would like to find all the records that have duplicate values in this column. What is the best query I can use to find the duplic

Answer 1

One very late contribution... in case it helps anyone waaaaaay down the line... I had a task to find matching pairs of transactions (actually both sides of account-to-account transfers) in a banking app, to identify which ones were the 'from' and 'to' for each inter-account-transfer transaction, so we ended up with this:

SELECT 
    LEAST(primaryid, secondaryid) AS transactionid1,
    GREATEST(primaryid, secondaryid) AS transactionid2
FROM (
    SELECT table1.transactionid AS primaryid, 
        table2.transactionid AS secondaryid
    FROM financial_transactions table1
    INNER JOIN financial_transactions table2 
    ON table1.accountid = table2.accountid
    AND table1.transactionid <> table2.transactionid 
    AND table1.transactiondate = table2.transactiondate
    AND table1.sourceref = table2.destinationref
    AND table1.amount = (0 - table2.amount)
) AS DuplicateResultsTable
GROUP BY transactionid1
ORDER BY transactionid1;

The result is that the DuplicateResultsTable provides rows containing matching (i.e. duplicate) transactions, but it also provides the same transaction id's in reverse the second time it matches the same pair, so the outer SELECT is there to group by the first transaction ID, which is done by using LEAST and GREATEST to make sure the two transactionid's are always in the same order in the results, which makes it safe to GROUP by the first one, thus eliminating all the duplicate matches. Ran through nearly a million records and identified 12,000+ matches in just under 2 seconds. Of course the transactionid is the primary index, which really helped.

Answer 2

Taking @maxyfc's answer further, I needed to find all of the rows that were returned with the duplicate values, so I could edit them in MySQL Workbench:

SELECT * FROM table
   WHERE field IN (
     SELECT field FROM table GROUP BY field HAVING count(*) > 1
   ) ORDER BY field

Answer 3

The following will find all product_id that are used more than once. You only get a single record for each product_id.

SELECT product_id FROM oc_product_reward GROUP BY product_id HAVING count( product_id ) >1

Code taken from : http://chandreshrana.blogspot.in/2014/12/find-duplicate-records-based-on-any.html

Answer 4

SELECT ColumnA, COUNT( * )
FROM Table
GROUP BY ColumnA
HAVING COUNT( * ) > 1

Answer 5

to get all the data that contains duplication i used this:

SELECT * FROM TableName INNER JOIN(
  SELECT DupliactedData FROM TableName GROUP BY DupliactedData HAVING COUNT(DupliactedData) > 1 order by DupliactedData)
  temp ON TableName.DupliactedData = temp.DupliactedData;

TableName = the table you are working with.

DupliactedData = the duplicated data you are looking for.

Answer 6

I saw the above result and query will work fine if you need to check single column value which are duplicate. For example email.

But if you need to check with more columns and would like to check the combination of the result so this query will work fine:

SELECT COUNT(CONCAT(name,email)) AS tot,
       name,
       email
FROM users
GROUP BY CONCAT(name,email)
HAVING tot>1 (This query will SHOW the USER list which ARE greater THAN 1
              AND also COUNT)