MySQL select records for duplicates using multiple columns

Question

I would like the select records from a table, or insert them into a new blank table where multiple of the columns is the same as another record in the database. The problem

Answer 1

If you want to count duplicates among multiple columns, use group by:

select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicates
from table
group by ColumnA, ColumnB, ColumnC

If you only want the values that are duplicated, then the count is bigger than 1. You get this using the having clause:

select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicates
from table
group by ColumnA, ColumnB, ColumnC
having NumDuplicates > 1

If you actually want all the duplicate rows returns, then join the last query back to the original data:

select t.*
from table t join
     (select ColumnA, ColumnB, ColumnC, count(*) as NumDuplicates
      from table
      group by ColumnA, ColumnB, ColumnC
      having NumDuplicates > 1
     ) tsum
     on t.ColumnA = tsum.ColumnA and t.ColumnB = tsum.ColumnB and t.ColumnC = tsum.ColumnC

This will work, assuming none of the column values are NULL. If so, then try:

     on (t.ColumnA = tsum.ColumnA or t.ColumnA is null and tsum.ColumnA is null) and
        (t.ColumnB = tsum.ColumnB or t.ColumnB is null and tsum.ColumnB is null) and
        (t.ColumnC = tsum.ColumnC or t.ColumnC is null and tsum.ColumnC is null)

EDIT:

If you have NULL values, you can also use the NULL-safe operator:

     on t.ColumnA <=> tsum.ColumnA and
        t.ColumnB <=> tsum.ColumnB and
        t.ColumnC <=> tsum.ColumnC