Hi, I have a few questions related to the the logical modelling section to discuss. Q1 . When there is {mandatory,AND} constraints, shouldn't we be putting flag attributes in the single relation? In the sample question it is not included. Q2. Is there a need to include alternate keys when deriving relations? Q3. When identifying whether attribute can be NULL or NOT NULL, Besides the primary keys, shouldn't we also include whether foreign keys could be NULL or NOT? Thank you for your help!

1. Someone also asked this question on the email. Since all books can be up for sale and loan, you do not need to include a flag. However, if you include a flag, that is also fine. 2. No, you do not need to define alternate keys (recall the lecture). You can if you want to. However, make sure to define the primary keys. 3. Regarding "Not NULL". You do need to mention primary keys (also note in sqlite, you specifically need to mention PK NOT NULL). Regarding FKs, it is possible in some cases to have a NULL FK . However, if there is a mandatory participation of a FK, in that case FK will be not null. Also with FKs do mention the actions for ON DELTE and ON UPDATE. Cheers

Thank you for the quick reply. In a case if a subclass/subclasses don't have their own attributes, then there would be a need to include flags. Is that right?