Question

i want to put unique index on two (or more) columns in a table, but i get “duplicate keys found”. How to select those rows which cause duplication?

Answer 1

You can use Group By and Having for this:

SELECT col1,
       col2
FROM   table
GROUP  BY col1,
          col2
HAVING Count(*) > 1

Basically – group the values, then filter for instances where there is more than one.