The Language of SQL- P9 pdf

The syntax for the previous statement in Oracle is: SELECT Orders.LastName FROM Orders123 Orders; Two remaining reasons for using table aliases will be covered in Chapters 11 and 14: ■ S

Table Aliases

In addition to providing alternate names for columns, aliases can also be speci-fied for tables, using the sameAS keyword There are three general reasons for using table aliases

The first reason relates to tables with obscure or complex names For example, if

a table is named Orders123, you can use the followingSELECTto give it an alias

of Orders

SELECT

LastName

FROM Orders123 AS Orders

Unlike column aliases, table aliases are not enclosed in quotes When using table aliases, you have the option of using the alias as a prefix for any selected columns For example, the above could also be written as:

SELECT

Orders.LastName

FROM Orders123 AS Orders

The prefix Orders has now been added as a prefix to LastName, using a period to separate the prefix from the column name In this situation, the prefix wasn’t necessary However, when data is selected from multiple tables, the prefix will sometimes be required This will be seen in later chapters

D A T A B A S E D I F F E R E N C E S : O r a c l e

In Oracle, table aliases are specified without the AS keyword The syntax for the previous statement

in Oracle is:

SELECT

Orders.LastName

FROM Orders123 Orders;

Two remaining reasons for using table aliases will be covered in Chapters 11 and 14:

■ Situations when selecting from multiple tables

■ Situations when using a subquery in aSELECTstatement

Chapter 3 ■ Calculations and Aliases

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The meaning of the term subquery will become clear in Chapter 14 when the topic

is covered in detail

Looking Ahead

In this chapter, you learned about three general ways to create calculated fields in

aSELECT statement First, literal values can be used to select specific words or

values Second, arithmetic calculations can be used to perform calculations on

single or multiple columns Third, concatenation can be used to combine

col-umns and literal values together We also discussed the related topic of column

aliases, which are often employed when using calculated fields

In the next chapter, we’ll be moving on to the subject of functions, which provide

a slightly more complex way to perform calculations As mentioned before, we’re

not quite at the point where you can apply selection criteria to your statements

I’m still building on the basics of what can be done with the columnlist in a

SELECT Don’t worry We’ll get to the exciting stuff soon enough In the

meantime, your patience in sticking with this methodical approach will soon

pay off

Looking Ahead 27

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chapter 4

Using Functions

Keywords Introduced: LEFT , RIGHT ,

SUBSTRING , LTRIM , RTRIM , CONCAT , UPPER ,

RAND , PI , CAST , ISNULL/IFNULL/NVL

For those of you familiar with spreadsheet software such as Microsoft Excel, you know that functions provide a huge amount of functionality for the typical spreadsheet user Without the ability to use functions, most of the data available

in spreadsheets would be of limited value The same is true in the world of SQL Your familiarity with some of the most commonly used SQL functions will greatly enhance your ability to generate dynamic results for those who will be using your reports

This chapter covers a wide variety of some of the most commonly used functions in four different categories: character functions, date/time functions, numeric tions, and conversion functions Additionally, we will talk about composite func-tions, which are a way of combining multiple functions into a single expression

The Function of Functions

Similar to the calculations covered in the previous chapter, functions provide another way to manipulate data As was seen, calculations involve multiple fields, either with arithmetic operators such as multiplication or by concatenation In contrast, functions are often performed on a single column

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What is a function? A function is merely a rule for transforming a value (or values) into another value, using a specific formula For example, the function

SUBSTRINGcan be used to determine that the first initial of the name JOAN is J

There are two types of functions: scalar and aggregate The term scalar comes

from mathematics and refers to an operation that is done on a single number In computer usage, it means that the function is performed on data in a single row For example, the LTRIM function removes leading spaces from one specified value

In contrast, aggregate functions are meant to be performed on a larger set of data For example, theSUM function can be used to calculate the sum of all the values of a specified column Since aggregate functions apply to sets or groups of data, we will leave our discussion of them to Chapter 10

Every SQL database offers dozens of scalar functions The functions vary widely between databases, in terms of their names and also how they work As a result,

we will only cover a few representative examples of some of the more useful functions

The most common types of scalar functions can be classified under three cate-gories: character, date/time, and numeric Obviously, these are functions that allow you to manipulate character, date/time, or numeric datatypes

In addition, you will learn about some useful conversion functions that can be used to convert data from one datatype to another

Character Functions

Character functions are those functions that enable you to manipulate character

data Just as character datatypes are sometimes called string datatypes, character functions are sometimes called string functions I’m going to cover these eight

examples of character functions:LEFT, RIGHT, SUBSTRING, LTRIM, RTRIM,

CONCAT,UPPER, andLOWER

In this chapter, rather than retrieve data from specified tables, I’m going to simply use SELECT statements with literal values Let’s start with our first example, which is for theLEFTfunction When you issue this SQL command:

SELECT

LEFT ('sunlight',3) AS 'The Answer'

Chapter 4 ■ Using Functions

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