（一）HQL
HQL：Hibernate Qusery Language，如果你已經熟悉它，就會發現它跟SQL非常相像。不過 你不要被表面的假象迷惑，HQL是面向對象的（OO，用生命的眼光看待每一個對象，他們是如此 鮮活）。如果你對JAVA和SQL語句有一定了解的話，那麼HQL對你簡直易如反掌，你完全可以利用在公車上的時間掌握它。以下從幾個方面進行慢慢深入：1。大小些敏感大家知道SQL-92 Query是對大小寫不敏感的，但是在HQL（前面提到它是OO的）中對對象類的名稱和屬性確實大小寫敏感的（符合Java編程語法）。
HQL 子句本身大小寫無關，但是其中出現的類名和屬性名必須注意大小寫區分如：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(參看上邊關於大小寫敏感時的例子情形)，如：from eg.Cat as cat 這裡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.Dogfrom eg.Cat as cat, eg.Dog as dog3。join相關(inner) joinleft (outer) joinright (outer) joinfull joinHQL同樣對SQL中的這些特性支持下面插播一個小話題，關於上邊的那些特性，我一直都沒怎麼用，今天既然說到這裡，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) joinselect 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 (注意到條件語句我用on 沒有用where)那麼執行結果是什麼呢？id1 name1 id2 name2++++++++++++++++++++++++++++++++++++++001 Jplateau 01 研發部002 Jony 01 研發部2).left (outer) joinselect employee.ID as id1,employee.Name as name1,department.ID as id2,department.Nameas name2 from Employee as employee left join Department as department on employee.DepNo=department.ID 那麼執行結果又該是什麼呢？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准，第二個表中沒有相應紀錄的時候填充null} 3). right (outer) joinselect employee.ID as id1,employee.Name as name1,department.ID as id2,department.Nameas name2 from Employee as employee right join Department as department on employee.DepNo=department.ID 那麼執行結果又該是什麼呢？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准，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.Nameas 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到底是做什麼用的？望給於說明） 等等
5。數學函數JDO目前好像還不支持此類特性。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（如果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=dog8。表達式在SQL語句中大部分的表達式在HQL中都可以使用：mathematical operators +, -, *, / 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，在這裡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裡的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，它的後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，但他們兩個骨子裡卻是一樣的，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語句沒什麼差別，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語句沒什麼差別，如：select employee.name,employee.DepNo from Employee as employee group by employee.DepNoselect 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 )（二）條件查詢Criteria Query
。數學函數JDO目前好像還不支持此類特性。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（如果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=dog8。表達式在SQL語句中大部分的表達式在HQL中都可以使用：mathematical operators +, -, *, / 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，在這裡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裡的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，它的後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，但他們兩個骨子裡卻是一樣的，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語句沒什麼差別，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語句沒什麼差別，如：select employee.name,employee.DepNo from Employee as employee group by employee.DepNoselect 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 )（二）條件查詢Criteria Query
Criteria criteria = session.createCriteria(TUser.class);criteria.add(Expression.eq("name", "Erica"));criteria.add(Expression.eq("sex", new Integer(1)));
