Desired behaviour:
1. If there is a row with 'ABC' (in a unique column) in the table, a row
with 'abc' should not be allowed
2. If I do SELECT * FROM aTable WHERE aColumn = 'ABC', I should see a row
with 'abc' as well (if there is one in the table)
This has been described in this mailing list a few months ago, in more
detail.