create barcode image c# Manipulating Database Data in Visual Basic .NET

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Manipulating Database Data
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ow that you know something about writing database queries, it s time to turn your attention to the different aspects of data modification, such as retrieving, inserting, updating, and deleting data. In this chapter, we ll cover the following: Retrieving data Using SELECT INTO statements Inserting data Updating data Deleting data
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A SQL query retrieves data from a database. Data is stored as rows in tables. Rows are composed of columns. In its simplest form, a query consists of two parts: A SELECT list, where the columns to be retrieved are specified A FROM clause, where the table or tables to be accessed are specified
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s We ve written SELECT and FROM in capital letters simply to indicate they re SQL keywords. SQL isn t Tip
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case sensitive, and keywords are typically written in lowercase in code. In T-SQL, queries are called SELECT statements, but the ISO/ANSI standard clearly distinguishes queries from statements. The distinction is conceptually important. A query is an operation on a table that produces a table as a result; statements may (or may not) operate on tables and don t produce tables as results. Furthermore, subqueries can be used in both queries and statements. So, we ll typically call queries queries instead of SELECT statements. Call queries whatever you prefer, but keep in mind that queries are a special feature of SQL.
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CHAPTER 5 s MANIPULATING DATABASE DATA
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Using two keywords, SELECT and FROM, here s the simplest possible query that will get all the data from the specified table: Select * from <table name>
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The asterisk (*) means you want to select all the columns in the table. You will be using a SQLEXPRESS instance of SQL Server 2005 in this chapter. Open SQL Server Management Studio Express and in the Connect to Server dialog box select <ServerName>\SQLEXPRESS as the server name and then click Connect. SQL Server Management Studio Express (SSMSE) will open. Expand the Databases node and select the Northwind database. Your screen should resemble that shown in Figure 5-1.
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Figure 5-1. Selecting a database to query
Try It Out: Running a Simple Query
To submit a query to retrieve all employee data, open a New Query window in SSMSE (remember to make Northwind your query context). Enter the following query and click Execute. You should see the results shown in Figure 5-2. Select * from employees
CHAPTER 5 s MANIPULATING DATABASE DATA
Figure 5-2. Query results pane
How It Works
You ask the database to return the data for all columns, and you get exactly that. If you scroll to the right, you ll find all the columns in the Employees table. Most of the time, you should limit queries to only relevant columns. When you select columns you don t need, you waste resources. To explicitly select columns, enter the column names after the SELECT keyword, as shown in the following query, and click Execute. Figure 5-3 shows the results. Select employeeid, firstname, lastname from employees This query selects all the rows from the Employees table but only the EmployeeID, FirstName, and LastName columns.
CHAPTER 5 s MANIPULATING DATABASE DATA
Figure 5-3. Selecting specific columns
Using the WHERE Clause
Queries can have WHERE clauses. The WHERE clause allows you to specify criteria for selecting rows. This clause can be complex, but we ll stick to a simple example for now. The syntax is as follows: WHERE <column1> <operator> <column2 / Value> Here, <operator> is a comparison operator (for example, =, <>, >, or <). (Table 5-1, later in the chapter, lists the T-SQL comparison operators.)
Try It Out: Refining Your Query
In this exercise, you ll see how to refine your query. 1. Add the following WHERE clause to the query in Figure 5-3. Where country = 'USA' 2. Run the query by pressing F5, and you should see the results shown in Figure 5-4.
CHAPTER 5 s MANIPULATING DATABASE DATA
Figure 5-4. Using a WHERE clause
s Caution SQL keywords and table and column names aren t case sensitive, but string literals (enclosed
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