EJB3 JPA JPQL 语句 查询 使用 函数 分页 子查询（2）

时间：2022-09-11 13:40:25
原文地址：EJB3 JPA JPQL 语句 查询 使用 函数 分页 子查询（2）作者：花籽的泪关联(join)JPQL仍然支持和SQL中类似的关联语法：left out join/left joininner joinleft join fetch/inner join fetch  left out join/left join等，都是允许符合条件的右边表达式中的Entiies为空（需要显式使用left join/left outer join的情况会比较少。）例：//获取26岁人的订单,不管Order中是否有OrderItemselect o from Order o left join o.orderItems where o.ower.age=26 order by o.orderid  inner join 要求右边的表达式必须返回Entities。例：//获取26岁人的订单,Order中必须要有OrderItemselect o from Order o inner join o.orderItems where o.ower.age=26 order by o.orderid  ！！重要知识点：在默认的查询中，Entity中的集合属性默认不会被关联，集合属性默认是延迟加载( lazy-load )。那么，left fetch/left out fetch/inner join fetch提供了一种灵活的查询加载方式来提高查询的性能。例：private String QueryInnerJoinLazyLoad(){// 默认不关联集合属性变量(orderItems)对应的表Query query = em.createQuery("select o from Order oinner join o.orderItems where o.ower.age=26 order by o.orderid");List result = query.getResultList();if (result!=null && result.size()>0){//这时获得Order实体中orderItems(集合属性变量)为空Order order = (Order) result.get(0);//当需要时，EJB3 Runtime才会执行一条SQL语句来加载属于当前Order的//OrderItemsSet<OrderItem> list =order.getOrderItems();Iterator<OrderItem> iterator = list.iterator();if (iterator.hasNext()){OrderItem orderItem =iterator.next();System.out.println ("订购产品名："+ orderItem.getProductname());}} 上面代码在执行"select o from Order o inner join o.orderItems where o.ower.age=26 order by o.orderid"时编译成的SQL如下（他不包含集合属性变量(orderItems)对应表的字段）：select order0_.orderid as orderid6_, order0_.amount as amount6_, order0_.person_id asperson4_6_, order0_.createdate as createdate6_ from Orders order0_ inner join OrderItemsorderitems1_ on order0_.orderid=orderitems1_.order_id, Person person2_ whereorder0_.person_id=person2_.personid and person2_.age=26 order by order0_.orderid  上面代码当执行到Set<OrderItem> list = order.getOrderItems();时才会执行一条SQL语句来加载属于当前Order的OrderItems，编译成的SQL如下：select orderitems0_.order_id as order4_1_, orderitems0_.id as id1_, orderitems0_.id as id7_0_,orderitems0_.order_id as order4_7_0_, orderitems0_.productname as productn2_7_0_,orderitems0_.price as price7_0_ from OrderItems orderitems0_ where orderitems0_.order_id=?order by orderitems0_.id ASC 这样的查询性能上有不足的地方。为了查询N个Order，我们需要一条SQL语句获得所有的Order的原始对象属性，但需要另外N条语句获得每个Order的orderItems集合属性。为了避免N+1的性能问题，我们可以利用join fetch一次过用一条SQL语句把Order的所有信息查询出来 例子//获取26岁人的订单,Order中必须要有OrderItemQuery query = em.createQuery("select o from Order oinner join fetcho.orderItems whereo.ower.age=26 order by o.orderid"); 上面这句HPQL编译成以下的SQL：select order0_.orderid as orderid18_0_, orderitems1_.id as id19_1_, order0_.amount asamount18_0_,order0_.person_id as person4_18_0_, order0_.createdate as createdate18_0_,orderitems1_.order_id as order4_19_1_, orderitems1_.productname as productn2_19_1_,orderitems1_.price as price19_1_, orderitems1_.order_id as order4_0__,orderitems1_.id as id0__from Orders order0_ inner join OrderItems orderitems1_ onorder0_.orderid=orderitems1_.order_id, Person person2_ whereorder0_.person_id=person2_.personid and person2_.age=26 order by order0_.orderid,orderitems1_.id ASC 上面由于使用了fetch,这个查询只会产生一条SQL语句，比原来需要N+1条SQL语句在性能上有了极大的提升  排除相同的记录DISTINCT使用关联查询，我们很经常得到重复的对象，如下面语句："select o from Order o inner join fetch o.orderItems order by o.orderid"当有N个orderItem时就会产生N个Order,而有些Order对象往往是相同的，这时我们需要使用DISTINCT关键字来排除掉相同的对象。例：select DISTINCTo from Order o inner join fetch o.orderItems order by o.orderid   比较Entity在查询中使用参数查询时，参数类型除了String,原始数据类型( int, double等)和它们的对象类型( Integer, Double等),也可以是Entity的实例。例：//查询某人的所有订单Query query = em.createQuery("select o from Order o whereo.ower =?1 order by o.orderid");Person person = new Person();person.setPersonid(new Integer(1));//设置查询中的参数query.setParameter(1,person);  批量更新(Batch Update)HPQL支持批量更新例：//把所有订单的金额加10Query query = em.createQuery("update Order as o set o.amount=o.amount+10");//update 的记录数int result = query.executeUpdate();  批量删除(Batch Remove)例：//把金额小于100的订单删除,先删除订单子项,再删除订单Query query = em.createQuery("delete from OrderItem item where item.order in(from Order as o where o.amount<100)");query.executeUpdate();query = em.createQuery("delete from Order as o where o.amount<100");query.executeUpdate();//delete的记录数  使用操作符NOT//查询除了指定人之外的所有订单Query query = em.createQuery("select o from Order o wherenot(o.ower =?1)order by o.orderid");Person person = new Person();person.setPersonid(new Integer(2));//设置查询中的参数query.setParameter(1,person);  使用操作符BETWEENselect o from Order as o where o.amount between 300 and 1000 使用操作符IN//查找年龄为26,21的Personselect p from Person as p where p.age in(26,21) 使用操作符LIKE//查找以字符串"li"开头的Personselect p from Person as p where p.name like 'li%' 使用操作符IS NULL//查询含有购买者的所有Orderselect o from Order as o where o.ower is [not] null 使用操作符IS EMPTYIS EMPTY是针对集合属性(Collection)的操作符。可以和NOT一起使用。注：低版权的Mysql不支持IS EMPTY//查询含有订单项的所有Orderselect o from Order as o where o.orderItems is [not] empty 使用操作符EXISTS[NOT]EXISTS 需要和子查询配合使用。注：低版权的Mysql不支持EXISTS//如果存在订单号为1的订单，就获取所有OrderItemselect oi from OrderItem as oi where exists (select o from Order o where o.orderid=1)//如果不存在订单号为10的订单，就获取id为1的OrderItemselect oi from OrderItem as oi where oi.id=1 andnot exists (select o from Order o where o.orderid=10)  字符串函数JPQL定义了内置函数方便使用。这些函数的使用方法和SQL中相应的函数方法类似。包括：1. CONCAT 字符串拼接2. SUBSTRING 字符串截取3. TRIM 去掉空格4. LOWER 转换成小写5. UPPER 装换成大写6. LENGTH 字符串长度7. LOCATE 字符串定位 例：//查询所有人员，并在姓名后面加上字符串"_foshan"select p.personid, concat(p.name, '_foshan') from Person as p//查询所有人员,只取姓名的前三个字符select p.personid, substring(p.name,1,3) from Person as p 计算函数HPQL定义的计算函数包括：ABS 绝对值SQRT 平方根MOD 取余数SIZE 取集合的数量 例：//查询所有Order的订单号及其订单项的数量select o.orderid, size(o.orderItems) from Order as o group by o.orderid//查询所有Order的订单号及其总金额/10的余数select o.orderid, mod(o.amount, 10) from Order as o  子查询子查询可以用于WHERE和HAVING条件语句中例：//查询年龄为26岁的购买者的所有Orderselect o from Order as o where o.ower in(select p from Person as p where p.age =26)  结果集分页有些时候当执行一个查询会返回成千上万条记录，事实上我们只需要显示一部分数据。这时我们需要对结果集进行分页，QueryAPI有两个接口方法可以解决这个问题：setMaxResults( ) 和setFirstResult( )。 setMaxResults方法设置获取多少条记录setFirstResult方法设置从结果集中的那个索引开始获取（假如返回的记录有3条，容器会自动为记录编上索引，索引从0开始，依次为0，1，2） 例：public List getPersonList(int max,int whichpage) {try{intindex = (whichpage-1) * max;Query query =em.createQuery("from Person p order by personid asc");List list = query.setMaxResults(max).setFirstResult(index).getResultList();em.clear();//分离内存中受EntityManager管理的实体bean，让VM进行垃圾回收returnlist;} catch (Exception e) {e.printStackTrace();return null;}}
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