Re: Database introduction

On 2014-01-27 12:56:04 +0000, Max said:

> I'm reading "The Manga Guide to Databases" as a light introduction to
> databases.
> http://tinyurl.com/phjkd2c
>
>
> Unfortunately I can't grasp division in a relational database..
>
> This is the page that explains it:
>
> http://digilander.libero.it/maxxam99/Pics/2014-01-27_132837.jpg
>
> From what it tells me I'd make the following table(1 column):
>
> Date:
> 3/5
> 3/10
> 3/5
>
>
> Clearly I didn't understand much..

Not your fault. The book doesn't really explain it well.

What the division does in this case is to determine the Date values for which it holds that *all* (not just one) combinations with the records in EXPORT appear in SALES TABLE.

For 3/5 this is the case since both
12 | The Kingdom of Minanmi | 3/5 23 | Alpha Empire | 3/5
appear in SALES TABLE.

For 3/10 to be in the output both
12 | The Kingdom of Minanmi | 3/10 23 | Alpha Empire | 3/10
should appear in SALES TABLE. But only the first appears.

For 3/21 to be in the output both
12 | The Kingdom of Minanmi | 3/21 23 | Alpha Empire | 3/21
should appear in SALES TABLE. But none of them appears.

etc.

• Jan Hidders