MySQL: Selecting non-unique records

Scenario: you want to select all records that have a non-unique value in a column. Use a subquery:

SELECT ... FROM ...
WHERE ... IN (
   SELECT ... FROM ...
   GROUP BY ... HAVING COUNT(*) > 1
);

So, to give an example, you want to select all rows from the table "books" that have a non-unique title:

SELECT * FROM books
WHERE title IN (
   SELECT title FROM books
   GROUP BY title HAVING COUNT(*) > 1
);