Полезная информация

Chapter 2. Queries

Table of Contents
2.1. Overview
2.2. Table Expressions
2.2.1. FROM clause
2.2.2. WHERE clause
2.2.3. GROUP BY and HAVING clauses
2.3. Select Lists
2.3.1. Column Labels
2.4. Combining Queries
2.5. Sorting Rows

2.1. Overview

A query is the process of retrieving or the command to retrieve data from a database. In SQL the SELECT command is used to specify queries. The general syntax of the SELECT command is

SELECT select_list FROM table_expression [sort_specification]

The following sections describe the details of the select list, the table expression, and the sort specification. The simplest kind of query has the form

SELECT * FROM table1;

Assuming that there is a table called table1, this command would retrieve all rows and all columns from table1. (The method of retrieval depends on the client application. For example, the psql program will display an ASCII-art table on the screen, client libraries will offer functions to retrieve individual rows and columns.) The select list specification * means all columns that the table expression happens to provide. A select list can also select a subset of the available columns or even make calculations on the columns before retrieving them; see Section 2.3. For example, if table1 has columns named a, b, and c (and perhaps others) you can make the following query:

SELECT a, b + c FROM table1;

(assuming that b and c are of a numeric data type).

FROM table1 is a particularly simple kind of table expression. In general, table expressions can be complex constructs of base tables, joins, and subqueries. But you can also omit the table expression entirely and use the SELECT command as a calculator:

SELECT 3 * 4;

This is more useful if the expressions in the select list return varying results. For example, you could call a function this way.

SELECT random();