Chapter 3: How to retrieve data from a single table

In this chapter, you’ll learn how to code SELECT statements that retrieve data from a single table.

An introduction to the SELECT statement

To help you learn to code SELECT statements, this chapter starts by presenting its basic syntax. Next, it presents several examples that will give you an idea of what you can do with this statement. Then, the rest of this chapter will teach you the details of coding this statement.

The basic syntax of the SELECT statement

Figure 3-1 presents the basic syntax of the SELECT statement. The syntax summary at the top of this figure uses conventions that are similar to those used in other programming manuals. Capitalized words are keywords that you have to type exactly as shown. In contrast, you have to provide replacements for the lowercase words. For example, you can enter a list of columns in place of select_list, and you can enter a table name in place of table_source.

Beyond that, you can choose between the items in a syntax summary that are separated by pipes (|) and enclosed in braces ({}) or brackets ([]). And you can omit items enclosed in brackets. If you have a choice between two or more optional items, the default item is underlined. And if an element can be coded multiple times in a statement, it's followed by an ellipsis (...). You'll see examples of pipes, braces, default values, and ellipses in syntax summaries later in this chapter. For now, if you compare the syntax in this figure with the coding examples in the next figure, you should easily see how the two are related.

The syntax summary in this figure has been simplified so that you can focus on the four main clauses of the SELECT statement: SELECT, FROM, WHERE, and ORDER BY. Most of the SELECT statements you code will contain all four of these clauses. However, only the SELECT and FROM clauses are required.

The SELECT clause is always the first clause in a SELECT statement. It identifies the columns that will be included in the result set. These columns are retrieved from the base tables named in the FROM clause. Since this chapter focuses on retrieving data from a single table, the FROM clauses in all of the statements shown in this chapter name a single base table. In the next chapter, though, you'll learn how to retrieve data from two or more tables.

The WHERE and ORDER BY clauses are optional. The ORDER BY clause determines how the rows in the result set are sorted, and the WHERE clause determines which rows in the base table are included in the result set. The WHERE clause specifies a search condition that's used to filter the rows in the base table. This search condition can consist of one or more Boolean expressions, or predicates. A Boolean expression is an expression that evaluates to True or False. When the search condition evaluates to True, the row is included in the result set.

In this book, I won't use the terms "Boolean expression" or "predicate" because I don't think they clearly describe the content of the WHERE clause. Instead, I'll just use the term "search condition" to refer to an expression that evaluates to True or False.

The simplified syntax of the SELECT statement

SELECT select_list
FROM table_source
[WHERE search_condition]
[ORDER BY order_by_list]

The four clauses of the SELECT statement

Clause	Description
SELECT	Describes the columns that will be included in the result set.
FROM	Names the table from which the query will retrieve the data.
WHERE	Specifies the conditions that must be met for a row to be included in the result set. This clause is optional.
ORDER BY	Specifies how the rows in the result set will be sorted. This clause is optional.

Description

• You use the basic SELECT statement shown above to retrieve the columns specified in the SELECT clause from the base table specified in the FROM clause and store them in a result set.
• The WHERE clause is used to filter the rows in the base table so that only those rows that match the search condition are included in the result set. If you omit the WHERE clause, all of the rows in the base table are included.
• The search condition of a WHERE clause consists of one or more Boolean expressions, or predicates, that result in a value of True, False, or Unknown. If the combination of all the expressions is True, the row being tested is included in the result set. Otherwise, it's not.
• If you include the ORDER BY clause, the rows in the result set are sorted in the specified sequence. Otherwise, the rows are returned in the same order as they appear in the base table. In most cases, that means that they're returned in primary key sequence.

Note: The syntax shown above does not include all of the clauses of the SELECT statement. You'll learn about the other clauses later in this book.

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Figure 3-1 The basic syntax of the SELECT statement

SELECT statement examples

Figure 3-2 presents five SELECT statement examples. All of these statements retrieve data from the Invoices table. If you aren't already familiar with this table, you should use the Management Studio as described in the last chapter to review its definition.

The first statement in this figure retrieves all of the rows and columns from the Invoices table. Here, an asterisk (*) is used as a shorthand to indicate that all of the columns should be retrieved, and the WHERE clause is omitted so that there are no conditions on the rows that are retrieved. Notice that this statement doesn't include an ORDER BY clause, so the rows are in primary key sequence. You can see the results following this statement as they're displayed by the Management Studio. Notice that both horizontal and vertical scroll bars are displayed, indicating that the result set contains more rows and columns than can be displayed on the screen at one time.

The second statement retrieves selected columns from the Invoices table. As you can see, the columns to be retrieved are listed in the SELECT clause. Like the first statement, this statement doesn't include a WHERE clause, so all the rows are retrieved. Then, the ORDER BY clause causes the rows to be sorted by the InvoiceTotal column in ascending sequence.

The third statement also lists the columns to be retrieved. In this case, though, the last column is calculated from two columns in the base table, CreditTotal and PaymentTotal, and the resulting column is given the name TotalCredits. In addition, the WHERE clause specifies that only the invoice whose InvoiceID column has a value of 17 should be retrieved.

The fourth SELECT statement includes a WHERE clause whose condition specifies a range of values. In this case, only invoices with invoice dates between 05/01/2008 and 05/31/2008 are retrieved. In addition, the rows in the result set are sorted by invoice date.

The last statement in this figure shows another variation of the WHERE clause. In this case, only those rows with invoice totals greater than 50,000 are retrieved. Since none of the rows in the Invoices table satisfy this condition, the result set is empty.

A SELECT statement that retrieves all the data from the Invoices table

SELECT *
FROM Invoices

(114 rows)

A SELECT statement that retrieves three columns from each row, sorted in ascending sequence by invoice total

SELECT InvoiceNumber, InvoiceDate, InvoiceTotal
FROM Invoices
ORDER BY InvoiceTotal

(114 rows)

A SELECT statement that retrieves two columns and a calculated value for a specific invoice

SELECT InvoiceID, InvoiceTotal, CreditTotal + PaymentTotal AS TotalCredits
FROM Invoices
WHERE InvoiceID = 17

A SELECT statement that retrieves all invoices between given dates

SELECT InvoiceNumber, InvoiceDate, InvoiceTotal
FROM Invoices
WHERE InvoiceDate BETWEEN '2008-05-01' AND '2008-05-31'
ORDER BY InvoiceDate

(29 rows)

A SELECT statement that returns an empty result set

SELECT InvoiceNumber, InvoiceDate, InvoiceTotal
FROM Invoices
WHERE InvoiceTotal > 50000

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Figure 3-2 SELECT statement examples