What does this point about first normal form mean?

Question

I'm reading the Wikipedia page on first normal form, and am confused about the last bullet point:

5) All columns are regular [i.e. rows have no hidden components such as row IDs, object IDs, or hidden timestamps].

Does this mean "all information about a row is contained in its values" (my words)?

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I understand the rest of the points about 1NF; it's only this one that I'm confused about.

Answer 1

I think that's basically true. The core of 1NF is that a table is strictly relational, i.e., all values in the table are some description of what the primary key identifies. Lots of RDBMSs use 'hidden' columns that don't describe the relation, they exist for some sort of internal housekeeping, yet are part of the table instead of being kept elsewhere in the schema (or some other mechanism the DB engine can use). Oracle, for instance, has 'hidden' rowID's everywhere.

The problem with this, as I understand it, is that because you have non-relational data skulking about your relations (i.e., tables), you create the possibility of reasoning about your data (i.e., creating SQL statements) which do not conform to relational algebra.

Answer 2

First normal form means that each column has exactly one meaningful thing in it. An easy way to tell is if: data from a single column is broken up into multiple values before it is used; or, conversely, if multiple values are concatenated and then stored in one column, then first normal form is broken.

This is a somewhat poor example, but I'm stuck on thinking of a better one. Storing names as a single column that is first and last name concatenated it would break 1NF, because you are storing more than a single meaningful value in that column. This is a poor example though because it's not 100% true, if names are always used as first+last together then it wouldn't break 1NF, though this generally isn't the case.

To address this point specifically, it is not part of Codd's original definition, its an addition that Date feels is important and is further explained in his book Date On Database. This basically means that the spirit of points 1-4 are not violated by special ways to access additional information about a row. RDMSs have started adding features that can violate this point if used.