The Language of SQL- P7 potx

The SELECTstatement would then look like: SELECT LastName FROM Customers and the resulting data would be: LastName Smith Lopez Harper If you want to select more than one, but not all col

Specifying Columns

So far, we’ve done nothing more than simply display all the data in a table But what if you wanted to select only certain columns? Working from the same table, you might want to display only the customer’s last name, for example The SELECTstatement would then look like:

SELECT LastName

FROM Customers

and the resulting data would be:

LastName

Smith

Lopez

Harper

If you want to select more than one, but not all columns, the SELECT might look like:

SELECT

FirstName,

LastName

FROM Customers

and the output would appear as:

FirstName LastName

William Smith

Natalie Lopez

Brenda Harper

The general format of this statement is:

SELECT columnlist

FROM table

The important thing to remember is that if you need to specify more than one

column in the columnlist, then those columns must be separated by commas.

Chapter 2 ■ Basic Data Retrieval

16

Also notice that we placed each column in the columnlist on separate lines This

was done to improve readability

Column Names with Embedded Spaces

What if a column contains a space in its name? Let’s say, for example, that the

LastName column was specified as Last Name instead (with a space inserted

between the two words) Clearly, the following would not work:

SELECT

Last Name

FROM Customers

This statement would be considered invalid since Last and Name are not column

names And even if Last and Name were proper column names, they would need

to be separated by a comma The solution is to use a special character around any

column name containing spaces The character to use differs, depending on

which database you’re using For Microsoft SQL Server, the characters to use are

square brackets, and the proper syntax is:

SELECT

[Last Name]

FROM Customers

One additional syntax note: Just as keywords are not case sensitive, it’s also true

that table and column names are not case sensitive For example, the previous

example is identical to:

select

[last name]

from customers

For clarity’s sake, I will print all keywords in all caps and capitalize table and

column names in this book, but that is not truly necessary

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

For MySQL, the character to use around column names containing spaces is an accent grave (`) The

syntax for the above example is:

SELECT

`Last Name`

FROM Customers;

Column Names with Embedded Spaces 17

For Oracle, the character to use around column names containing spaces is the double quote The syntax for the example is:

SELECT

"Last Name"

FROM Customers;

Additionally, unlike Microsoft SQL Server and MySQL, column names surrounded by double quotes are case sensitive That means that the previous statement is not equivalent to:

SELECT

"LAST NAME"

FROM Customers;

Looking Ahead

In this chapter, we’ve begun our exploration of how to use the SELECT state-ment to retrieve data We learned about basic syntax and have seen how to select specific columns In reality, however, this allows us to accomplish very little of a practical nature Most significantly, we have not yet learned how to apply any type of selection criteria to our data retrieval efforts For example, while we know how to select all customers, we don’t yet know how to only select customers from the state of New York

As it happens, I won’t be covering selection criteria until Chapter 7 What will we

be doing until then? In the next few chapters, we’re going to build on what can be

done with the columnlist component of the SELECT statement In the next chapter, I’ll be moving on to more variations on column selection, allowing us to create complex calculations in a single column I’ll also be talking about ways to rename columns to make them more descriptive

Similarly, Chapters 4 through 6 will build on your ability to create even more

complex and powerful columnlists, so when we finally get to the topic of selection

criteria, you will have a full arsenal of techniques available at your disposal

Chapter 2 ■ Basic Data Retrieval

18

chapter 3

Calculations and

Aliases

Keyword Introduced: AS

The topics covered in this chapter will allow you to present information in a more convenient and interesting format for anyone viewing your data The main

technique to be discussed is known as calculated fields This technique will allow

you to perform calculations on individual data items that you retrieve from a database

Using this approach, customer names can be formatted exactly as desired Numeric calculations specific to your business or organization can be made and presented As a SQL developer, you often need the ability to customize the con-tent of individual columns in order to successfully turn data into more intelligent content Calculated fields are very useful tools that can help you accomplish that goal

Calculated Fields

When selecting data from a table, you are not restricted to the columns that happen to be in the table The concept of calculated fields allows for a number of other possibilities With calculated fields, you can do the following:

■ Select specific words or values

■ Perform calculations on single or multiple columns

■ Combine columns and literal values together

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Let’s look at a few examples, all coming from this Orders table:

OrderID FirstName LastName QuantityPurchased PricePerItem

Literal Values

Our first example of a calculated field isn’t really a calculation at all We’re going

to select a specific value as a column, even though the literal value has nothing to

do with data in the table This type of expression is called a literal value Here’s an

example:

SELECT

'First Name: ',

FirstName

FROM Orders

This statement will return this data:

(no column name) FirstName

First Name: William

First Name: Natalie

First Name: Brenda

In this statement, we are selecting two data items The first is the literal value 'First Name:' Note that single quote marks are used to indicate that this is a literal with character data The second data item is the FirstName column Notice two things First, the literal ‘First Name’ is repeated on every row Second, there is no header information for the first column When run in Microsoft SQL Server, the column header displays (‘‘no column name’’) The reason why there is

no header is simply because this is a calculated field There isn’t a column name that can be used for the header

Chapter 3 ■ Calculations and Aliases

20