Chapter 4. SQL Syntax 第四章 SQL語法 This chapter describes the syntax of SQL. It forms the foundation for understanding the following chapters which will go into detail about how SQL commands are applied to define and modify data. 該章講述了SQL語法。為理解後面對於SQL命令如何定義和修改資料的詳解章節奠定基礎。   We also advise users who are already familiar with SQL to read this chapter carefully because it contains several rules and concepts that are implemented inconsistently among SQL databases or that are specific to PostgreSQL. 同樣建議已經熟悉SQL的使用者也認真的讀一下本章，因為本章中包含一些與SQL資料庫不一致的、獨屬於PostgreSQL的規則和概念等。