Duplicates in two columns in a table

thor-in-the-poppies-lisa-twede

That’s a painting of my cat Thor in the backyard.  Apparently he’s looking at the hose, despite the fact that to his right the yard is full of California Poppies.

Today’s blog has some code that is not my own.  Michael Powaga posted it over on Stack Overflow.  I just wanted to keep track of this little gem because I’ve needed it twice now and both times I ended up going with his solution.

I needed to know how many rows (and which ones) were duplicates but not just on one column, on two columns.  I had a situation where (I feel) there should have been a constraint to prevent duplicates in this table.  I wanted to neatly identify the offending records so I could deal with them.

This is his code.  It’s at the link above, but also copied here just in case.

select s.id, t.* 
from [stuff] s
join (
    select name, city, count(*) as qty
    from [stuff]
    group by name, city
    having count(*) > 1
) t on s.name = t.name and s.city = t.city
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