In addition to what everyone else has said, I would also tighten up your use of NULLs. Setting your columns up as NOT NULL by default is a good practice to have. Not that using NOT NULL moves your tables into a new normal form, but it does help identify possible normalization areas. I've seen, too often, developers using NULLable columns in tables because not all rows in the table use that column. This type of use of NULLs usually means that the entity is not normalized because you've got a number of columns that don't depend on the key.

It does not look like this is the case for your tables, but a good practice is to always start off with columns defined as NOT NULL and only change it if there is a good business reason to allow NULL values in that column.