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/Clean Up/SQL”

The SELECT operationin SQL can be broken down into three smaller operations,

which we will referto as the SELECT statement,the SELECT expression,

and the SELECTquery.

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HAVING

GROUP BY—When youuse aggregate functions in the SELECT clause

to produce summaryinformation, you use the GROUP BY clause to

divide theinformation into distinct groups.Your database system uses

any column or listof columns following the GROUP BY keywords as

grouping columns.The GROUP BY clause is optional, and we’ll examine

it further inChapter 13, Grouping Data.

HAVING—The HAVINGclause filters the result of aggregate functions

in groupedinformation. It is similar to the WHERE clause in that the

HAVING keyword isfollowed by an expression that evaluates to true,

false, orunknown.You can test the expression by using standard comparison

operators, Booleanoperators, or special operators. HAVING is

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

Filtering GroupedData.