The Language of SQL- P6 pptx

But it was basically still the case that data needed to be processed one record at a time.. The main problem with data storage prior to relational databases was not how the data was stor

In the 1970s, advances were made as hierarchical and network databases were

invented and utilized These newer databases, through an elaborate system of

internal pointers, made it easier to read through data For example, a program

could read a record for a customer, automatically be pointed to all orders for that

customer, and then be pointed to all details for each order But it was basically

still the case that data needed to be processed one record at a time

The main problem with data storage prior to relational databases was not how

the data was stored, but how it was accessed The real breakthrough with

rela-tional databases came when the language of SQL was developed, because it

allowed for an entirely new method of accessing data

Unlike earlier data retrieval methods, SQL permitted the user to access a large set

of data at a time With one single statement, a SQL command could retrieve or

update thousands of records from multiple tables This eliminated a great deal of

complexity Computer programs no longer needed to read one record at a time

in a special sequence, while deciding what to do with each record What used to

require hundreds of lines of programming code could now be accomplished with

just a few lines of logic

Looking Ahead

This chapter has provided enough background information about relational

databases so that you can move on to the main topic, which involves retrieving

data from databases We have discussed a number of important characteristics of

relational databases, such as primary keys, foreign keys, and datatypes We also

have talked about the possible existence of NULL values in data We will add to

our discussion of NULL values in Chapter 8 and return to the general topics of

database maintenance in Chapter 18 and database design in Chapter 19

Why is the all-important topic of database design held off until much later in the

book? In the real world, databases are designed and created before any data

retrieval is attempted Why would I not follow the same sequence of events in

this book? In short, I have found that it is much more productive to plunge into

using SQL without having to worry about details of database design In truth,

database design is as much an art as it is a science As such, the principles of

database design will be much more meaningful after you’re more aware of the

details and nuances of retrieving some data So we’re going to temporarily ignore

the question of how to design a database and jump right into data retrieval in our

very next chapter

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chapter 2

Basic Data

Retrieval

In this chapter, we are going to begin our exploration of the most important topic in SQL: how to retrieve data from a database Whether you’re in a large or small organization, the most common request made of SQL developers is the request for a report Of course, it’s a nontrivial exercise to get data into a data-base But once data is in a database, the energies of business analysts turn to the wealth of data at their disposal and the desire to extract useful information from all that data This is where the fun and usefulness of SQL begins

The emphasis in this book on data retrieval corresponds nicely to the real-world demands that are placed on SQL developers Your typical analysts don’t care about how data gets into a database, but they do care about how to get some-thing out of it Your knowledge of SQL will go a long way toward helping your organization unlock the secrets of the data stored in their databases

A Simple SELECT

The ability to retrieve data in SQL is accomplished through something called the

SELECTstatement Without a lot of preliminary explanation, here is an example

of the simplest possibleSELECTstatement:

SELECT * FROM Customers

In the SQL language, as in all computer languages, certain words are keywords.

These words have a special meaning and must be used in a particular way In this

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statement, the wordsSELECT andFROM are keywords The SELECT keyword indicates that you are beginning aSELECTstatement

The FROM keyword is used to designate the table from which data is to be retrieved The name of the table follows theFROM In this case, the table name is Customers

As is the custom, I will print keywords in all capital letters This is done to ensure that they are noticeable

The asterisk (*) in this example is a special symbol that means ‘‘all columns.’’

So to sum up, the statement means: Select all columns from the Customers table

If the Customers table looks like this:

CustomerID FirstName LastName

1 William Smith

2 Natalie Lopez

3 Brenda Harper

then thisSELECTwill return the following data:

CustomerID FirstName LastName

1 William Smith

2 Natalie Lopez

3 Brenda Harper

In other words, it brings back everything in the table

In the first chapter, I mentioned that it’s a common practice to specify a primary key for all tables In the previous example, the CustomerID column is such a column I also mentioned that primary keys are sometimes set up to generate sequential numbers automatically in a numeric sequence as rows are added to a table This is the case in the previous example Most of the sample data I’ll show throughout this book will show a similar column that is both a primary key and

defined as auto-increment By convention, this is generally the first column in a

table

Chapter 2 ■ Basic Data Retrieval

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Syntax Notes

Two points must be remembered when writing any SQL statement First, the

keywords in SQL are not case sensitive The wordSELECTis treated identically

to ‘‘select’’ or ‘‘Select.’’

Second, a SQL statement can be written on any number of lines For example, the

SQL statement:

SELECT * FROM Customers

is identical to:

SELECT *

FROM Customers

It’s usually a good idea to begin each important keyword on a separate line

When we get to more complex SQL statements, this will make it easier to quickly

grasp the meaning of the statement

Finally, as I present different SQL statements in this book, I will often show both

a specific example and a more general format For instance, the general format of

the previous statement would be shown as this:

SELECT *

FROM table

Italics are used to indicate a general expression The italicized word table means

that you can substitute any table name of your own in that spot So when you see

italicized words in any SQL statement in this book, that is simply my way of

saying that you can put any valid word or phrase in that location

D A T A B A S E D I F F E R E N C E S : M y S Q L a n d O r a c l e

Many SQL implementations require a semicolon (;) at the end of every statement This is true of

MySQL and Oracle, but not of Microsoft SQL Server For simplicity, I will show SQL statements

without semicolons in this book If you’re using MySQL or Oracle, you’ll need to add a semicolon to

the end of your statements Therefore, the previous statement would appear as:

SELECT *

FROM Customers;

Syntax Notes 15