Re: Database introduction

On 27/01/2014 15:49, Jan Hidders wrote:
> 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

now it's very clear, I'll print your answer! ;-) Received on Mon Jan 27 2014 - 16:16:49 CET