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.