Hibernate的HQL基础

a）大小写敏感问题：只对类和属性区分大小写。
	b）关联：
		(inner) join                
		left (outer) join
		right (outer) join
		full (outer) join
		
		with
			from Emp e join Dept d on e.deptId = d.id
			with e.name is not null			  
			
		fetch：把所关联的对象一同取出来	
			from Emp e join fetch e.dept
			select e.dept from Emp e
		
		from Emp e
		where e.dept.company.group.address.city is not null
		
	1)	select子句：
		select u.name, u.score from User u；
		select new User(u.name, u.score) from User u；
		select new List(u.name, u.score) from User u;
		select new Map(u.name as uname, u.score as uscore) from User u;
 	
 	2）命名参数
 		形式 —— “:paraname”	
 		query.setParameter("paraname", "z");	
 		
	3）where子句
		a、比较运算：可以比较对象
				from Emp e1, Emp e2
				where e1.dept = e2.dept;
		b）内置class属性
			from Product p where p.class = Book;		
		  	相当于 from Book;
		  	
	4）内置函数
		a）字符串：
				连接  ‘a’ || ‘b’
				upper(s)、lower(s)
				trim(s)
				length(s)
				substring(s, start, len)		
		b）日期函数：
				取当前日期（时间）：current_date(),current_time(),current_timestamp()
				处理日期字段的：year(date_field),month(date_field),day(date_field)	,hour(),minute(),second()			
				  		from Order o 
				  		whrere year(o.createDate) = 2010	
		c）类型转换函数：
				str(age)   str(createDate)
				cast(age as string)
				cast('2011-01-05' as date)
				
				'2011-01-05 12:10:23'
		d）条件分支——“if else”
			case category when 1 then 'Book'
			when 2 then 'DVD'
			else 'unknown'
			end	
			
			case when money_val <= 20000 then 'deptManager'
			when money_val >= 20000 and money_val <= 50000 then 'fz'
			else 'topleader'
			end	

         5）子查询（嵌套查询）
		from Cat fatcat
			where fatcat.weight >
							(select avg(weight) from Cat);
	
         6）注意：某些底层数据库不支持某些特殊的功能。
			如 having中聚集函数（sum）在某些数据库	（如MySql）是不支持的。

				from Emp e
				group by e.age
				having avg(e.age) < 60