title: Database normalization and duplicate values
body: <p>Consider a Parent / Child / GrandChild  structure in a database table schema, or even a deeper hierarchy. These being in the same aggregate. One table DAYS keeps a single row per day, and has a "Date" field. This is the root table, or maybe a child of the root. No row can ever be deleted in this table. </p>
  
  <p>In this case, however complex my table schema looks like, however far away in the hierarchy any other table is, <strong>is there any reason why any other table would hold a Date value?  Can't it instead just have a FK to the DAYS table.</strong> </p>
  
  <p>I obviously assume that the creation of these date fields happen not before such datefield exist in the DAYS table.</p>
  
  <p>I'm now thinking just about the date part to be relevant, not the time part. Not sure if all databases can store these individually. That's maybe relevant, but not really the main focus of the question. </p>