1。大小些敏感 大家知道Query是對大小寫不敏感的，但是在HQL（前面提到它是OO的）中那么對象類的名稱和屬性確實大小寫敏感的（符合java編程語法）。 如：sElect cat.name from Cat as cat和select cat.name from Cat as cat是一樣的 但是： sElect cat.name from CAT as cat和select cat.name from Cat as cat確實不一樣的。
2。from語句 最簡單的： from eg.Cat 它只是簡單的返回所有eg.Cat的實例 通常我們此時會為eg.Cat其個別名，因為在query的其余部分可能會用到(參看上邊關于大小寫 敏感時的例子情形)，如： from eg.Cat as cat 這里as可以省略。 上邊只是單表查詢，多表的情況如下寫法： from eg.Cat,eg.Dog from eg.Cat as cat,eg.Dog as dog
3。join相關 (inner) join left (outer) join right (outer) join full join HQL同樣對SQL中的這些特性支持 下面插播一個小話題，關于上邊的那些特性，我一直都沒怎么用，今天既然說到這里，就想 把上邊的幾個特性的用法說一下，也算對自己的一個補充： 假設有兩個表：部門、員工，下面列舉一些數據： 員工(Employee)： ID Name DepNo 001 Jplateau 01 002 Jony 01 003 Camel 02 部門(Department)： ID Name 01 研發部 02 營銷部
在Hibernate中我們操縱的都是對象，所以我們操縱的是部門類和員工類 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 (注重到條件語句我用on 沒有用where) 那么執行結果是什么呢？ id1 name1 id2 name2 ++++++++++++++++++++++++++++++++++++++ 001 Jplateau 01 研發部 002 Jony 01 研發部
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 那么執行結果又該是什么呢？ id1 name1 id2 name2 ++++++++++++++++++++++++++++++++++++++ 001 Jplateau 01 研發部 002 Jony 01 研發部 003 Camel null null {就是說此時我要已第一個表的記錄多少為準，第二個表中沒有相應紀錄的時候填充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 那么執行結果又該是什么呢？ id1 name1 id2 name2 ++++++++++++++++++++++++++++++++++++++ 001 Jplateau 01 研發部 002 Jony 01 研發部 null null 02 營銷部 {就是說此時我要已第二個表的記錄多少為準，第一個表中沒有相應紀錄的時候填充null}
4。select語句 就是要確定你要從查詢中返回哪些對象或者哪些對象的屬性。寫幾個例子吧： 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？望給于說明） 等等 5。數學函數 JDO目前似乎還不支持此類特性。 avg(...), sum(...), min(...), max(...)
count(*)
count(...), count(distinct ...), count(all...)
其用法和SQL基本相同
select distinct employee.name from Employee as employee select count(distinct employee.name),count(employee) from Employee as employee
6。polymorphism (暫時不知道如何解釋？) from com.test.Animal as animal 不光得到所有Animal得實例，而且可以得到所有Animal的子類（假如我們定義了一個子類Cat） 一個比較極端的例子 from java.lang.Object as o 可以得到所有持久類的實例
7。where語句 定義查詢語句的條件，舉幾個例子吧： from Employee as employee where employee.Name='Jplateau' from Employee as employee where employee.Name like 'J%' from Employee as employee where employee.Name like '%u' 在where語句中“=”不光可以比較對象的屬性，也可以比較對象，如： select animal from com.test.Animal as animal where animal.name=dog
binary comparison operators =, >=, <=, <>, !=, like
logical operations and, or, not
string concatenation
SQL scalar functions like upper() and lower()
Parentheses ( ) indicate grouping
in, between, is null
JDBC IN parameters ?
named parameters :name, :start_date, :x1 （這種應該是另一種"?"的變通解決方法）
SQL literals 'foo', 69, '1970-01-01 10:00:01.0'
Java public static final constants eg.Color.TABBY
其他不必解釋了，在這里我只想對查詢中的參數問題說明一下： 大家知道在SQL中進行傳遞參數進行查詢的時候，我們通常用PReparedStatement，在語句中寫一大堆的“？”， 在hql中也可以用這種方法，如： List mates = sess.find( "select employee.name from Employee as employee " + "where employee.Name=? ", name, Hibernate.STRING ); (說明：上面利用session里的find方法，在hibernate的api Session中重載了很多find方法，它可以滿足你多種形式的查詢) 上邊是一個參數的情形，這種情況下緊接著引入參數和定義參數的類型，當為多個參數，調用另一個find方法，它的后兩個 參數都是數組的形式。
還有另外一種方法來解決上邊的問題，JDO也有這樣的方法，不過和hibernate的表現形式上有差別，但他們兩個骨子里卻是 一樣的，如： Query q = sess.createQuery("select employee.name from Employee as employee where employee.Name=:name"); q.setString("name", "Jplateau"); //當有多個參數的時候在此逐一定義 Iterator employees = q.iterate();
9。order 語句 和sql語句沒什么差別，如： select employee.name from Employee as employee where employee.Name like 'J%' order by employee.ID desc (或者asc)
10。group by 語句 同樣和sql語句沒什么差別，如：
select employee.name,employee.DepNo from Employee as employee group by employee.DepNo
select foo.id, avg( elements(foo.names) ), max( indices(foo.names) ) from eg.Foo foo group by foo.id {Note: You may use the elements and indices constrUCts inside a select clause, even on databases with no subselects.} 誰幫我解釋一下上邊兩句，謝過！
11。子查詢 hibernate同樣支持子查詢，寫幾個例子：
from eg.Cat as fatcat where fatcat.weight > ( select avg(cat.weight) from eg.DomesticCat cat )
