doubts about a constraint's definition

Hello:
I've the following question:

I need to create a column, where the information is conserved in the same
way that it was written by the user, but the data of this column is unique
if it's only taken to capital

declaring a constraint
... unique(column name) is not enough to me  'cause

'ab'
'AB'
'aB'
....
will be accepted





Thanks in advance for any help
Enediel
Linux user 300141

Happy who can penetrate the secret causes of the things
¡Use Linux!