问题
I was looking at this video on normalization on youtube and I have to say I am confused now, I there might be errors in the video even though it has 25 likes and only 1 dislike.
Normalization
Specifically regarding the 2NF section which can be found at the 5 minute mark.
The author says the 'Assignment Description' column depends on the Assignment ID column...yet there are there are different Assignment Descriptions for the same Assignment ID. He then 'normalizes' this table into 2NF by breaking it into two tables...one of them containing only Assignment ID and Assignment Description. And this table is not even in 1NF (non unique primary key) so how can it be 2NF?
Can someone tell me if this video is correct or incorrect regarding 2NF?
回答1:
From the relational point of view, that video is, at best, sloppy and careless. Some parts of it are simply wrong, including the part you cite. Don't watch any more videos from the "Database Normalization Master".
For a relation to be in 2NF it must
- be in 1NF, and
- have no partial key dependencies.
If an attribute depends on part of any candidate key, then there's a partial key dependency, and the relation is not in 2NF.
One point the video tries to make is that if the primary key is a single column, the relation is guaranteed to be in 2NF. But look at this table (from Wikipedia entry for 2NF).
Employees' Skills
--
Employee Skill Current Work Location
Jones Typing 114 Main Street
Jones Shorthand 114 Main Street
Jones Whittling 114 Main Street
Bravo Light Cleaning 73 Industrial Way
Ellis Alchemy 73 Industrial Way
Ellis Flying 73 Industrial Way
Harrison Light Cleaning 73 Industrial Way
This relation has only one candidate key, {Employee, Skill}. But there's a partial key dependency between Employee and Current Work Location. Given any value for Employee, we find one and only one value for Current Work Location.
Changing the primary key to a surrogate ID number won't change the situation, although the video implies it will.
Employees' Skills
--
ID Employee Skill Current Work Location
1 Jones Typing 114 Main Street
2 Jones Shorthand 114 Main Street
3 Jones Whittling 114 Main Street
4 Bravo Light Cleaning 73 Industrial Way
5 Ellis Alchemy 73 Industrial Way
6 Ellis Flying 73 Industrial Way
7 Harrison Light Cleaning 73 Industrial Way
The primary key is a single column, which they say guarantees this relation is in 2NF. But it isn't. The video makers ignore the fact that most tables have multiple candidate keys, and that normalization takes every candidate key into account, not just the "special" candidate key called the primary key.
There are a lot of other problems with this video--so many that I didn't watch it all the way through. Life's too short to waste another minute on it.
来源：https://stackoverflow.com/questions/15166890/learning-database-normalization-confused-about-2nf