PARTII: SQL Basics

CHAPTER 4Creating a Simple Query

介绍一种怎样创建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.

一层包括一层,相互嵌套使用,能够构成很复杂的SELECT.

SELECT Statement的其他关键词都比較熟悉,这儿略微解释GROUP
BY和HAVING

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.

HAVING—The HAVING clause filters the result of aggregate functions

in grouped information. It is similar to the WHERE clause in that the

HAVING 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. HAVING is

also an optional clause, and we’ll take a closer look at it in Chapter 14,

Filtering Grouped Data.