Build Your Own ASP.NET 2.0 Web Site Using C# & VB, Second Edition

Using the SELECT Statement

The most common of all SQL queries is the SELECT query. This query is generally

constructed using a SELECT clause and a FROM clause. To understand this concept

more clearly, take a look at the following statement, which retrieves all columns

of all records in the Departments table:

SELECT *

FROM Departments

In this case, the SELECT clause lists the columns that you want to retrieve. In this

case, we used *, which means “all columns.” The FROM clause specifies the table

from which you want to pull the records. Together, these two clauses create an

SQL statement that extracts all data from the Departments table.

You’ve probably noticed that the two clauses appear on separate lines. If you

wanted to keep the entire statement on one line, that’s fine, but SQL lets you

separate the statements on multiple lines to make complex queries easier to read.

Also note that although SQL is not actually a case-sensitive language, we’ll capit-

alize the keywords (such as SELECT and FROM) according to the popular convention.

To sum up, here’s the basic syntax used in a SELECT query:

SELECT

This keyword indicates that we want to retrieve data, rather than modify,

add, or delete data—these activities use the UPDATE, INSERT, and DELETE

keywords, respectively, in place of SELECT.

columns

We must provide the names of one or more columns in the database table

from which we want to retrieve data. We can list multiple columns by separ-

ating the column names with commas, or we can use * to select all columns.

We can also prefix each column name with the table name, as shown here:

SELECT Employees.Name, Employees.Username

FROM Employees.Name

This approach is mandatory when two or more of the tables we’re dealing

with contain columns that have the same names. We’ll learn to read data

from multiple tables a little later in the chapter.

297

Using the SELECT Statement

FROM

The FROM keyword ends the SELECT clause and starts the FROM clause, which

identifies the tables from which the data will be extracted. This clause is re-

quired in all SELECT statements.

tables

We need to identify the names of the tables from which we want to extract

data. To list multiple tables, separate their names with commas. Querying

multiple tables is called a table join—we’ll cover this a bit later.

Armed with this knowledge, we can see that the preceding sample statement

would retrieve all records from the Departments table, producing a set of results

like that shown in Figure 8.4.

Figure 8.4. Reading the list of departments

See how easy it is? The SELECT query is probably the one you’ll use most.

The Number of Affected Rows

As you can see in Figure 8.4, SQL Server reports the number of records that

have been affected by a certain query. This report doesn’t indicate that those

records were modified. Instead, the figure represents the number of rows

that were read, modified, deleted, or inserted by a certain query.

298

Chapter 8: Speaking SQL