Your approach is insecure.  If you modify some value in the database, you'll be encrypting a new value using the same IV as you used to encrypt the prior value.  (The data will be stored in the same place, so the table name, column name, and primary key will remain unchanged.)  This might compromise confidentiality.  The compromise is especially bad if you are using a stream cipher or AES-CTR or similar mode, as then you've encrypted two different values using the same stream, a classic keystream-reuse vulnerability (the two-time pad is highly insecure).

Instead, it is better to do as others recommend: generate a random IV, and store it together with the ciphertext.  Indeed, in most modes of operation, you can consider the IV to be part of the ciphertext.

AES using derived keys / IVs. Does it introduce a weakness?
I'm using Windows XP and I hate the long start up time when I shut down and then turn on my laptop. However if I hibernate instead of shutting down it starts up much faster. Are there any harmful effects / disadvantages if I always hibernate whenever I'm done for the day instead of shutting down?
