SQL weakness and errors on database processing at SPAM LINK

I want to share with you all some practical posts related to:

Problems with EXISTS, ALL and INTERSECT
Meanings of null

in this site SPAM LINK

And soon the complete papers.

thanks, oswaldo, i moved your thread to the Database Concepts forum

please note: many of us on this forum are very happy with sql, and some of us are especially fond of nulls

so be careful, okay?

hi,

i am facing some problem on using nulls value, can you please advice what is the thing we need to consider when use allow null field for comparison (during join) and sum...

GongXi, please show your details

because to answer your question based on the information you've provided, the answer is: "you need to consider that NULL is not equal to anything, not even another NULL, and that aggregate functions like SUM() ignore NULLs"

In the Database Concepts forum we can discuss this, but it will be at a pretty high (theoretical) level. I sense from the way you phrased your question that you are more interested in the practical details.

As r937 suggested, we really need some specific details to work from unless you really want "database platitudes" that go into the theoretical basis of handling NULL values within SQL.

-PatP

I am sorry for not my question clear.

I am not sure whether my table design in correct way or not.
For example, we have one table which contain my customer company information.
Example:
Table Name: Customer
CustCode
CustName
CustAddress
CustPhone

and our customer some will have branch, and we use another table to store
Table Name: CustBranch
CustCode
BranchCode
BranchName
BranchAddress
BranchPhone

Our customer are able to post their advertisement for their HQ or Branch, we have one table to store all this information
Table Name: CustAds
CustCode (foreign key)
BranchCode(foreign key, allow null)
AdsDate
AdsDetail

for those advertisment which is post by HQ, we set null for BranchCode.

how i can list all advertisment info with HQ/Branch address & phone?
Means, if the advertisement is post for HQ then show Customer info, if is post for Branch, then show branch information.

however, i really want to know what is the advantage of using null, i still new to all this, so i hope to learn more.

very appreciate on your help.

it is clear enough, don't worry

it is good

like this --
you're doing fine

Hi,

I advise you to see the post "SOME MEANINGS OF NULL" at www.SPAMLINK.com, there you can find some helpful hints about nulls.

ofique, perhaps you should give the more specific link Some Meanings of NULL instead

your article basically says that

1. NULL can be either "missing" or "inapplicable"

2. withdrawals not made through an ATM can't have an ATM foreign key and therefore it must be NULLable

3. bank accounts that do not belong to a bank (??) cannot be included in queries which compute account balances per bank

i think your article should actually go further, and give suggestions for fixing situations 2. and 3.

In "SOME MEANINGS OF NULL" the point is the following:

In SQL we should have a way to differentiate null, I mean the result of a SQL query should be divided into three one for TRUE result, other for FALSE result and, of course, another for null result, whatever this mark means in the context.

maybe the examples in the article are a little confusing but I think the point mentioned is important.

Thanks

i certainly agree with you, understanding NULL is very important

have you seen this? Nulls - A Socratic Enquiry

GonXi,

I've just taken a quick look at your table structure, and something strikes me as... odd

It's regarding your table "CustBranch"
The name (to me) suggests that it should be an interim table in a many-to-many relationship between customers and branch. The design suggests (see last parenthesis) that one customer can have one branch, which again, just seems odd!

Hello:

I want to share with you all an investigation I made related to come of the SQL weaknesses and interesting characteristics of this language on database processing in the following site:

www.SPAMLINK.com

All the examples in this sites has been verified with DB2, the situation is practically the same in other DBMS.I think that the aspects analysed in this site are essential for people who know SQL.

I hope it’s useful.

we frown on people doing "drive-by" postings with "throwaway" usernames -- you register, spam us with your link, go away, then come back, re-register, and spam us again.

please go back to using ofigue

this thread is now merged with your previous one

thanks for understanding