数据库知识：hibernate查询语句--HQL

1 .from

1.1单表查询

from eg.cat as cat.其中，cat只是一个别名，为了用其他子语句的时候书写简单

1.2多表查询

from eg.Cat,eg.Dog

from eg.Cat as cat,eg.Dog as dog

2 join相关

(inner) join

left (outer) join

right (outer) join

full join

HQL同样对SQL中的这些特性支持

下面插播一个小话题，关于上边的那些特性，我一直都没怎么用，今天既然说到这里，就想

把上边的几个特性的用法说一下，也算对自己的一个补充：

假设有两个表：部门、员工，下面列举一些数据：

员工(Employee)：
ID Name DepNo
001 Jplateau 01
002 Jony 01
003 Camel 02
部门(Department)：
ID Name
01 研发部
02 营销部

在Hibernate中我们操纵的都是对象，所以我们操纵的是部门类和员工类

1).(inner) join

select employee.ID as id1,employee.Name as name1,department.ID as id2,department.Name

as name2 from Employee as employee join Department as department on employee.DepNo=

department.ID (注意到条件语句我用on 没有用where)

那么执行结果是什么呢？

id1 name1 id2 name2

++++++++++++++++++++++++++++++++++++++

001 Jplateau 01 研发部

002 Jony 01 研发部

2).left (outer) join

select employee.ID as id1,employee.Name as name1,department.ID as id2,department.Name

as name2 from Employee as employee left join Department as department on employee.DepNo=

department.ID

那么执行结果又该是什么呢？

id1 name1 id2 name2

++++++++++++++++++++++++++++++++++++++

001 Jplateau 01 研发部

002 Jony 01 研发部

003 Camel null null

{就是说此时我要已第一个表的记录多少为准，第二个表中没有相应纪录的时候填充null}

3). right (outer) join

select employee.ID as id1,employee.Name as name1,department.ID as id2,department.Name

as name2 from Employee as employee right join Department as department on employee.DepNo=

department.ID

那么执行结果又该是什么呢？

id1 name1 id2 name2

++++++++++++++++++++++++++++++++++++++

001 Jplateau 01 研发部

002 Jony 01 研发部

null null 02 营销部

{就是说此时我要已第二个表的记录多少为准，第一个表中没有相应纪录的时候填充null}

3 大小写敏感

4。select语句

就是要确定你要从查询中返回哪些对象或者哪些对象的属性。写几个例子吧：

select employee form Employee as employee

select employee form Employee as employee where employee.Name like 'J%'

select employee.Name form Employee as employee where employee.Name like 'J%'

select employee.ID as id1,employee.Name as name1,department.ID as id2,department.Name

as name2 from Employee as employee right join Department as department on employee.DepNo=

department.ID

select elements(employee.Name) from Employee as employee

（不明白elements到底是做什么用的？望给于说明）

等等

5。数学函数

JDO目前好像还不支持此类特性。

avg(...), sum(...), min(...), max(...)

count(*)

count(...), count(distinct ...), count(all...)

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