SQLkiwi (7/5/2011)
David Portas (7/5/2011)
...But it's the same relation variable as before! The atomicity of the string and the structure of the relation has not changed. Just because I intend to use it differently T cannot suddenly stop being a relation...Is it just me, or does this sound like a quantum theory of normal forms? :laugh:
To me It reads like two or more people are re-writing/debating the original RDBMS theory material. Trying to fit things Chris Date or Ted Codd wrote about to there own personal thoughts or ideas. I am not a big fan. People come here to learn. They should not be given the personal thoughts abouts someone elses ideas as a fact. That is misinformation.
An actual Quote from Chris Date: "There was a general feeling at the time that research belonged to everyone. It was public. People at IBM didn’t like it, but all the papers were available. Ellison felt it was obvious that Codd was onto something, so he built a clone of what Ted outlined"
If you want to keep writing about what you "think" Codd & Date meant please read some of the things they wrote yourself before posting. Try to stay away from what Oracle or IBM re-wrote later. Post some live links to that factual information along with your thoughts.
Since we have several examples in this thread of how not to do that. Here are some examples of how to do it:
For anyone that want's to read what Ted Codd wrote about NULL (SQL):
http://en.wikipedia.org/wiki/Null_(SQL) I know it is wikipedia, but so what if it is correct.
For anyone that want's to read what Codd & Date wrote about 1NF:
http://www2.yk.psu.edu/~lxn/IST_210/normal_form_definitions.html
FYI: The main definition for 1NF in the link above has the word DOMAIN in it. This is how Ted and later Chris both saw it.
FYI: Chris Date did not work with Ted Codd on the original definition of NULL. Chris came into the picture later.
It is the RDBMS Ternary Value theory Codd introduced that most people seem to confuse the most. Codd never said NULL = Unknown.
The Unknown truth value is generated whenever Null is compared with any data value, or with another Null.
Codd saw the need for differant levels of NULL in a RDMS to store and return the lack of a value properly. In 1990 Codd wrote in The Relational Model for Database Management version 2
"The single Null mandated by the SQL standard is inadequate, and should be replaced by two separate Null-type markers to indicate the reason why data is missing. These two Null-type markers are commonly referred to as 'A-Values' and 'I-Values', representing 'Missing But Applicable' and 'Missing But Inapplicable', respectively."
NULL, just like the mostly empty void we call the universe, is a large subject.:cool: