SQL Server Reference Guide

Mar 28, 2003

␡

⎙ Print

< Back Page 102 of 365 Next >

In our last tutorial (Database Objects: Tables&), we learned that tables are columns and rows of data. In any Relational Database Management System (RDBMS), the tables are atomic – that is, they only store the data that is pertinent to the table. Recall that I began the discussion with a selection from two tables, called authors and titles from the pubs database. We'll use those tables again.

In the earlier lesson, I explained the basics of relating the seemingly unconnected tables. I'll continue that line of thought in this article.

I also introduced the concept of relational algebra. Let's take a look at that concept again, and also combine a little relational calculus as well.

You may remember from school that relational algebra deals with sets of numbers, and the way they do or don't join. If you recall, each set (sometimes called a relation) contains values, and those values are called domains. That holds true in SQL databases as well. The other concept that holds true for both SQL Server and relational algebra is the way the sets of data join. Here are a few of those concepts:

The first thing to remember from relational algebra is that the results from any join form another set. In SQL, the results of any join end up as another data-set, in a table-like structure. This idea of creating smaller (or larger) results from a select statement will help you immensely as we move forward.

A select is the act of pulling out certain values from a set of data. SQL implements this concept with the SELECT statement, which you'll use quite often. Select statements are across rows or records of data and result in another data-set. We'll see how to fully form a SQL SELECT statement in a bit.

A union combines the results of two or more sets of data. The results of a union always create a larger data set. SQL Server also supports a UNION qualifier on a SELECT statement.

An intersection of data results in a listing of the common values of two or more sets. This is the most common type of operation that you'll use in SQL, since you will often want to know things like "which books were written by which authors?" We'll use this concept to build our tables, and also to build lots of queries as we move on.

A difference is the operation that shows the items in one set that do not exist in the others. SQL Server supports this type of operation by several methods, which we'll see a bit later.

Now we move to the relational calculus operations. Unless you had to suffer through a math discipline in your college degree as I did, you may not have been exposed to these concepts yet by these names, but I'll bet you'll understand them intuitively. They are:

Exists: The exists clause results in a true or false state when a value is in a set of data. SQL will perform the action requested when an EXISTS clause is used and the data is located in a SELECT statement. The action won't be implemented when the value isn't in the set. There is an opposite of this clause in SQL, called NOT EXISTS.

In: The in clause is similar to the exists clause, but isn't used to test for a true or false condition, but merely to include values in the action requested. There is an opposite of this clause in SQL, called NOT IN.

Between: The between clause is very useful and does pretty much what its name implies; it tests for true and false by including the values between two conditions. The immediate question is "inclusive or not?" and the answer is inclusive. If you're looking for a non-inclusive set, then you would use the > and < symbols. There is an opposite of this clause in SQL, called NOT BETWEEN.

As you can see, we have some powerful tools to work with. To make them even more powerful, you can combine these statements to create some fairly complex conditions. (We'll do that in future tutorials.) For now, the important thing is to have the ideas well in hand.

So far, we've only seen the way we select data from sets. We still need to cover the relationships of those sets.

Relating tables

To begin our discussion of table relationships, we need to define the various relationship types. We'll follow these concepts up with implementations as we design our databases.

The first type of relationship between tables isn't between two tables at all – it's the self-relationship. It's important to keep in mind that a single table really isn't single – in concept, anyway. It's an un-tapped source of data sets. There are situations in which you have a table with data that relates to other data in the table. For instance, an employee table has a domain (there's that algebra again!) which would contain all employees. Everyone who receives a paycheck would be considered an employee.

How would we handle defining each employee's manager? We might have another table for that, but it's unnecessary to go to all that trouble. We can include a reference to the employee number to whom a certain employee reports, right in the same table. We can then use SQL statements to relate the information right back to another row in the same table.

The next type of join is the one-to-one join. This condition exists if a slice of data in one set has one and only one match in another set. This type is actually pretty rare, sometimes seen in order-fulfillment systems.

The next type of join is the one-to-many relationship. This type of relationship is the most common. It states that there are many items in one or more sets that relate to another. For instance, one manager may manage many employees, but normally one employee doesn't have many managers! OK, bad example...

We saw the next condition in the last tutorial: the many-to-many relationship. In this type, two or more sets of data are related to each other more than once on both sides. For instance, many authors might work on a single book, and many books might be written by a single author.

Now that you have these concepts in hand, you can take real-world examples of data sets from your work and plug them into the join and selection types described above. It's helpful to ask these questions as you move forward on your design:

How does the process in question work? For instance, you might say "A seller sells items from inventory." Right away you see several sets of data (sellers, buyers, sales, items, inventory, etc.) and relationships (sales to items, items to inventory, sellers to buyers, etc.). These define your tables.
In how many ways are each set related to each other? There are really two questions here. How are the sets related, and how many times each? This is your relationship design phase. I'll show you a handy tool to discover the answers, the Entity Relationship Diagram (ERD), in another tutorial.
What is required? You need a seller to sell, a buyer to buy, and items in inventory to sell. The answers here determine the NULL conditions and also helps define the keys for your constraints.

What makes each set unique? We'll deal with this question more in the next tutorial where I'll describe keys more fully.

These are the basic concepts of the relationship puzzle. In future articles, I'm going to springboard this base knowledge into logical and physical database design, keys, select logic, capacity and performance planning and more. You can see these are important base concepts, which we'll build on for the rest of our programming series. Keep this reference handy!

< Back Page 102 of 365 Next >

🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Privacy Notice

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
Such marketing is consistent with applicable law and Pearson's legal obligations.
Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

As required by law.
With the consent of the individual (or their parent, if the individual is a minor)
In response to a subpoena, court order or legal process, to the extent permitted or required by law
To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
To investigate or address actual or suspected fraud or other illegal activities
To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020

Email Address