Part II. The SQL Language

第二部分：SQL语言

This part describes the use of the SQL language in PostgreSQL. We start with describing the general syntax of SQL, then explain how to create the structures to hold data, how to populate the database, and how to query it. The middle part lists the available data types and functions for use in SQL commands. The rest treats several aspects that are important for tuning a database for optimal performance. 

本部分讲述了PostgreSQL中的SQL语言使用。从讲述SQL基本语法开始，然后解释如何创建存储数据的结构，如何填充数据库，以及如何查询数据。中间部分列出了SQL命令中可用的数据类型及函数等。剩余部分讲述了性能调优的几个方面。

The information in this part is arranged so that a novice user can follow it start to end to gain a full understanding of the topics without having to refer forward too many times. The chapters are intended to be self-contained, so that advanced users can read the chapters individually as they choose. The information in this part is presented in a narrative fashion in topical units. Readers looking for a complete description of a particular command should see Part VI.

本部分中的相关信息，旨在让初学者在从头到尾学习之后可以获得对于相关主题的全面了解。该章节设置为独立的章节，以便高级用户可以根据需要选择阅读。本章内容以主题单元的叙述方式呈现。读者如果想要查看特定命令的详细用法，需查看第六部分。

Readers of this part should know how to connect to a PostgreSQL database and issue SQL commands. Readers that are unfamiliar with these issues are encouraged to read Part I first. SQL commands are typically entered using the PostgreSQL interactive terminal psql, but other programs that have similar functionality can be used as well.

阅读此部分需要首先具备连接PostgreSQL数据库及执行SQL语句的能力。如果对这两项能力有些生疏，建议首先阅读一下第一部分。SQL命令通常使用PostgreSQL的交互终端psql执行，但也可以使用其他具有类似功能的程序处理。