Normalization is always done at design level.
once the relationships are defined nothing can be done using sql - plus.
it is just the means to ask/give data from/to database.
if ur trying for query tunning, then there r possibilities.
Normalizaton "should" always be done at design level but modifications can be caried out on the database using SQL. Dropping table columns etc all have an effect on the data although I would not suggest it unless you know what you are doing. You can continue to redefine your database after the initial design and SQL is the tool used to make these changes.