15.6. select 子句

select 子句选择将哪些对象与属性返回到查询结果集中。考虑如下情况：

select mate
from Cat as cat
    inner join cat.mate as mate

该语句将选择其它 Cat 的 mate（其他猫的配偶）。实际上，你可以更简洁的用以下的查询语句表达相同的含义：

select cat.mate from Cat cat

查询语句可以返回值为任何类型的属性，包括返回类型为某种组件（Component）的属性：

select cat.name from DomesticCat cat
where cat.name like 'fri%'

select cust.name.firstName from Customer as cust

查询语句可以返回多个对象和（或）属性，存放在 Object[] 队列中，

select mother, offspr, mate.name
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

或存放在一个 List 对象中：

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

假设类 Family 有一个合适的构造函数 - 作为实际的类型安全的 Java 对象：

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

你可以使用关键字 as 给“被选择了的表达式”指派别名：

select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat

这种做法在与子句 select new map 一起使用时最有用：

select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat

该查询返回了一个 Map 的对象，内容是别名与被选择的值组成的名-值映射。