Question

I need to represent a form, called E-Report, where users will add data.

Every E-Report will have a QAP and every QAP will have two or more Defects.

These defects will show here, in this table:

At first time, this table will have two defects and user could add more defects from QAP, or insert new ones if he/she think that it is necessary.

User will check or not CRS, CRF, MA and MI columns and I will save these data on EReportDefect table.

By the way, QAP and Defect data will be in a web service, I need to download it to my Android device. I can’t modify these two tables adding more data.

To represent that I have designed this Visio:

Because I can’t add data to Defect table I’ve created EReportDefect table to store CRS, CRF, MA and MI user data and to store new Defects created by user.

To represent a new Defect added by user, I’ve set EReportDefect.defectId as NULL and EReportDefect.description to store new Defect description.

These new defects will have no value on EReportDefect.defectId and a value on EReportDefect.description.

Is this correct? Could I have a Foreign key column with a NULL value? Do you know a better approach?

Answer 1

Nullable foreign keys are perfectly acceptable.

Consider the following case, a messages table which is designed to be responded to by a user.

CREATE TABLE Messages 
(
  MessageId int,
  MessageText varchar(256),
  AnsweredByUserId int
)

Now, when the initial unanswered message is first created, AnsweredByUserId will be NULL. That’s perfectly fine. The message hasn’t been answered, so we wouldn’t expect to find a value in there.

When someone does answer the message, we want to put their UserId into AnsweredByUserId, and we want to enforce referential constraints when that happens ( e.g. we want to ensure that the UserId we insert actually exists in our Users table ).

In conclusion, fine to have a NULLABLE foreign key in the right circumstances.