This paper is written within the context of a relational data base model as presented by E. F. Codd in [1]. It also serves as a companion paper to his paper [2]. The central thesis is that a file operation should not produce unexpected 'side effects' in order to maintain a restriction (such as one-one, or many-one) on the file. Any file operation which would violate a restriction for some possible data is called 'violative'. It is determined which cases of the file operations WRITE, REWRITE and DELETE are violative. We argue that most violative file operations should be rejected. Instead, the equivalent operations should be carried out on projections in E. F. Codd's 'third normal form' in order to alleviate the necessity for spurious side effects. It is therefore convenient for the programmer to be able to refer directly to these projections by name and for the data management system to allow operations on them. This provides further justification to that in [2] for modelling the data base interface between programmer and data management system in terms of relations in 'third normal form'.