c# generate 2d barcode WRITING SIMPLE SELECT QUERIES in Font

Maker PDF 417 in Font WRITING SIMPLE SELECT QUERIES

CHAPTER 2 WRITING SIMPLE SELECT QUERIES
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Summary
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The SELECT statement is used to retrieve data from tables stored in SQL Server databases. The statement can be broken down into several parts called clauses. The FROM clause specifies the table where the data is stored. The SELECT clause contains a list of columns to be retrieved. To filter the data, use the WHERE clause. To sort the data, use the ORDER BY clause. This chapter covered a lot of ground, especially all the nuances of the WHERE clause. Make sure you understand the material covered in the chapter well before continuing. Everything you learn throughout the rest of the book will depend on a thorough knowledge of the basics. The next chapter explores many of the built-in functions you can use to make data retrieval even more interesting.
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CHAPTER 3
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Using Functions and Expressions
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Now that you have the knowledge to write simple SELECT statements, it is time to explore some of the other features of T-SQL that allow you to manipulate how the data is displayed, filtered, or ordered. To create expressions in T-SQL, you use functions and operators along with literal values and columns. The reasons for using expressions in T-SQL code are many. For example, you may want to display only the year of a column of the DATETIME data type on a report, or you may need to calculate a discount based on the order quantity in an order-entry application. Any time the data must be displayed, filtered, or ordered in a way that is different from how it is stored, you can use expressions and functions to manipulate it. You will find a very rich and versatile collection of functions and operators available to create expressions that manipulate strings and dates and much more. You can use expressions in the SELECT, WHERE, and ORDER BY clauses as well as in other clauses you will learn about in 5.
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Expressions Using Operators
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You learned how to use several comparison operators in the WHERE clause in 2. In this section, you will learn how to use operators to concatenate strings and perform mathematical calculations in T-SQL queries.
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Concatenating Strings
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The concatenation operator (+) allows you to add together two strings. The syntax is simple: <string or column name> + <string or column name>. Start up SQL Server Management Studio if it is not already running, and connect to your development server. Open a new query window, and type in and execute the code in Listing 3-1. Listing 3-1. Concatenating Strings USE AdventureWorks2008; GO --1 SELECT 'ab' + 'c'; --2 SELECT BusinessEntityID, FirstName + ' ' + LastName AS "Full Name" FROM Person.Person;
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CHAPTER 3 USING FUNCTIONS AND EXPRESSIONS
--3 SELECT BusinessEntityID, LastName + ', ' + FirstName AS "Full Name" FROM Person.Person; Figure 3-1 shows the results of Listing 3-1. Query 1 shows that you can concatenate two strings. Queries 2 and 3 demonstrate concatenating the LastName and FirstName columns along with either a space or a comma and space. Notice that you specified the alias, Full Name, to provide a column header for the result of the expressions combining FirstName and LastName. If you did not provide the alias, the column header would be (No column name), as in query 1.
Figure 3-1. The results of queries concatenating strings
Concatenating Strings and NULL
In 2 you learned about the challenges when working with NULL in WHERE clause expressions. When concatenating a string with a NULL, NULL is returned. Listing 3-2 demonstrates this problem. Type the code in Listing 3-2 into a new query window, and execute it. Listing 3-2. Concatenating Strings with NULL Values USE AdventureWorks2008; GO SELECT BusinessEntityID, FirstName + ' ' + MiddleName + ' ' + LastName AS "Full Name" FROM Person.Person;
CHAPTER 3 USING FUNCTIONS AND EXPRESSIONS
Figure 3-2 shows the results of Listing 3-2. The query combines the FirstName, MiddleName, and LastName columns into a Full Name column. The MiddleName column is optional; that is, NULL values are allowed. Only the rows where the MiddleName value has been entered show the expected results. The rows where MiddleName is NULL return NULL.
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