SQL查询初学者指南读书笔记(二)创建SQL查询

PARTII: SQL Basics

CHAPTER 4Creating a Simple Query

介绍一种如何创建SQL语句的技术--“Request/Translation/CleanUp/SQL”

The SELECT operation in SQL can be broken down into three smaller operations,
which we will refer to as the SELECT statement,the SELECT expression,
and the SELECT query.
一层包含一层,相互嵌套使用,可以构成非常复杂的SELECT.






SELECT Statement的其它关键词都比较熟悉,这儿稍微解释GROUP
BY和H***ING
GROUP BY—When you use aggregate functions in the SELECT clause
to produce summary information, you use the GROUP BY clause to
divide the information into distinct groups.Your database system uses
any column or list of columns following the GROUP BY keywords as
grouping columns.The GROUP BY clause is optional, and we’ll examine
it further in Chapter 13, Grouping Data.

H***ING—The H***ING clause filters the result of aggregate functions
in grouped information. It is similar to the WHERE clause in that the
H***ING keyword is followed by an expression that evaluates to true,
false, or unknown.You can test the expression by using standard comparison
operators, Boolean operators, or special operators. H***ING is
also an optional clause, and we’ll take a closer look at it in Chapter 14,
Filtering Grouped Data.