Is un-normalised data always contained in a single table? - mysql

Question

For an assignment I have created a database driven web application. I have to show my understanding of normalisation by showing my database in de-normalised form, and then normalising it gradually, explaining what was done at each stage.
The normalisation process at stages 1 to 3 (which is as far as we have to go) I have no trouble understanding.
My database contains 20+ tables and I don't know how I am supposed to represent this is 0NF. The main difficulty is due to the fact that, as I have understood, 0NF data is in a single table. In fact, I don't see any way around this because 0NF has no primary keys, and therefore there would be no way to reference data in other tables.
Am I right in thinking this? Or can I represent 0NF data in multiple tables, which would make this task a lot easier as I wouldn't have a 100+ column table.

Answer 1

0NF is a single table - like a spreadsheet of data. You wouldn't reference any other tables, you would simply repeat the data in the one table.
For example, imagine a messaging system:
Customer | Recipient | Message
Bob John Hello John
John Bob Hello
Bob John Have you got time to answer a question?
John Bob No way
We don't have a table containing the Person to link to, we repeat Bob or John in the customer column and in the recipient column.

Answer 2

0NF data can occur in multiple tables, each of which may be 0NF, but one table for everything is the worst form.
This may very well be the case of an assignment where you first have to fuck up your spontaneous solution first, so you can show the process of how to make it better.

Answer 3

You mean "unnormalized" not "de-normalized". The latter is when normalized base tables are replaced by others whose values are always the join of the orignals. You need to find out from whoever gave you the asignment whether unnormalized form here means your first design attempt or specifically a "universal relation" that is an appropriate join of all those. That would be de-normalizing.
Every base table and query result holds the rows that make some predicate (statement parameterized by columns) into a true proposition (statement).
SELECT * FROM EMP "employee [E] is named [N] and has dependent [D]"
SELECT * FROM DEP "employee [E] works for department [D]"
query SELECT E, N FROM EMP
for some D, "employee [E] is named [N] and has dependent [D]"
("employee [E] is named [N] and has some dependent")
An SQL FROM makes a temporary table that you can think of as having columns T.C for each column C of each table T. For inner JOINs (ie INNER, CROSS and plain) this temporary table is a cross join. It's predicate is the AND of the predicates of the joined tables. ON and WHERE conditions are also ANDed into the predicate. The SELECT clause renames the temporary columns so there are no "."s. (Athough SQL does that implicity if there's no ambiguity.)
query SELECT EMP.E AS E, N, DEP.D AS D FROM EMP JOIN DEP
"for some EMP.D, employee [EMP.E] is named [EMP.N] and has dependent [EMP.D]"
AND "employee [DEP.E] works for department [DEP.D]"
(ie "employee [E] is named [N] and has some dependent and works for department [D]")
Note that it doesn't matter what constraints hold. (Including UNIQUE, PRIMARY KEY,FOREIGN KEY & CHECK). Constraints just tell you that tables are limited in the values they will ever hold. In fact the constraints are determined by the predicates and the situations that can arise.
If you know that it's always the case that T1.C = T2.C for some column C of tables T1 & T2 then you only have to SELECT one of them, AS C. If every column C is always equal in every table then NATURAL JOIN does the appropriate = and the AS without having to mention any columns.
(More re predicates & SQL.)
PS The single-base version of a database is not a base whose value is the FULL ( OUTER ) JOIN of separate bases. First, normalization does not deal with NULLs, so you would have to remove them from any OUTER JOIN result, more or less giving you your tables back. Second, FULL JOIN is in general not associative, ie (T1 FULL JOIN T2) FULL JOIN T3 <> T1 FULL JOIN (T2 JOIN T3), so there is no such thing as "the FULL JOIN` of more than two tables". Third, even with just two tables their FULL JOIN does not in general allow you to reconstruct their values.
PPS There is no "0th Normal Form". There are different uses of "1st Normal Form". Sometimes it just means being a relation, and sometimes it means being a relation with no relation-valued attributes, and it is also frequently used in various other confused/nonsensible ways that are really about aspects of good design.