jpa 查询列表_终极JPA查询和技巧列表

jpa 查询列表

我们可以在Internet上找到一些JPA“如何做”，在本博客的此处，教您如何使用JPA执行多项任务。

通常，我看到有人问有关使用JPA进行查询的问题。通常，为了回答此类问题，提供了几个链接以尝试找到该问题的解决方案。

直到今天，我仍然找不到一个博客文章，其中汇集了有关使用JPA查询，性能/利用技巧，要下载的源代码的良好主题……

接受挑战

今天我们将看到：

模型类和将生成数据库数据的类
查找方法；使用getReference方法可获得更好的性能，并使用log4j在控制台中显示查询参数
JPQL：具有简单参数或对象的查询，联接，排序依据，在关系中导航
JPQL：功能：AVG，COUNT，MAX，MIN，TRIM，SUM，UPPER，LOWER，MOD，LENGHT，SQRT；使用HAVING，GROUP BY
JPQL：筛选条件：LIKE，IN，DISTINCT，EMPTY，BETWEEN，NULL，MEMBER OF，EXISTS（子查询），ANY，ALL，SOME，CONCAT，CURRENT_DATE，CURRENT_TIME，CURRENT_TIMESTAMP，LOCATE，SIZE，SUBSTRING
JPA：NamedQuery，使用日期查询，有关getSingleResult方法的警告
JPA：NativeQuery，名为NativeQuery
JPA：复杂的本地查询
JPA：使用EJB优化查询
JPA：分页
JPA：数据库提示
JPA：通过查询创建对象
JPQL：批量更新和删除
JPA：条件

您将看到在每个主类中，我们将调用方法“ CodeGenerator.generateData（） ”。此类方法仅在数据库中创建数据；有了这些数据，我们的查询将找到正确的结果。

在这篇文章的最后一页中，您会找到下载该文章源代码的链接。

在本文中，我们将使用带有Hibernate的JPA 2.0作为提供程序。该数据库将是HSQLDB，并将附加到该项目。您可以下载源代码并运行项目，而无需任何其他配置。我们不会谈论如何设置HSQLDB，因为本文的重点是如何查询数据库的数据。

这篇文章在某些方面不会使用开发的最佳实践。这篇文章的重点是展示JPA查询的工作方式。

模型类和将生成数据库数据的类

package com.model;import java.util.ArrayList;
import java.util.List;import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;@Entity
public class Person {@Id@GeneratedValue(strategy = GenerationType.AUTO)private int id;private String name;private int age;public Person() {}public Person(String name, int age) {this.name = name;this.age = age;}@OneToMany(mappedBy = 'person', cascade = CascadeType.ALL)private List<Dog> dogs;@OneToOne(cascade = CascadeType.ALL)@JoinColumn(name='address_id')private Address address;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public List<Dog> getDogs() {if (dogs == null) {dogs = new ArrayList<Dog>();}return dogs;}public void setDogs(List<Dog> dogs) {this.dogs = dogs;}public Address getAddress() {return address;}public void setAddress(Address address) {this.address = address;}@Overridepublic int hashCode() {return getId();}@Overridepublic boolean equals(Object obj) {if (obj instanceof Person) {Person person = (Person) obj;return person.getId() == getId();}return false;}
}

package com.model;import java.util.Date;import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;@Entity
public class Dog {@Id@GeneratedValue(strategy = GenerationType.AUTO)private int id;private String name;private double weight;@Temporal(TemporalType.TIMESTAMP)private Date dateOfBirth;public Dog() {}public Dog(String name, double weight, Date dateOfBirth) {this.name = name;this.weight = weight;this.dateOfBirth = dateOfBirth;}public static void main(String[] args) {System.out.println(new Date());}@ManyToOneprivate Person person;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getWeight() {return weight;}public void setWeight(double weight) {this.weight = weight;}public Date getDateOfBirth() {return dateOfBirth;}public void setDateOfBirth(Date dateOfBirth) {this.dateOfBirth = dateOfBirth;}public Person getPerson() {return person;}public void setPerson(Person person) {this.person = person;}@Overridepublic int hashCode() {return getId();}@Overridepublic boolean equals(Object obj) {if (obj instanceof Dog) {Dog dog = (Dog) obj;return dog.getId() == getId();}return false;}
}

package com.model;import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;@Entity
public class Address {@Id@GeneratedValue(strategy = GenerationType.AUTO)private int id;private String streetName;private int houseNumber;public Address() {}public Address(String streetName, int houseNumber) {this.streetName = streetName;this.houseNumber = houseNumber;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getStreetName() {return streetName;}public void setStreetName(String streetName) {this.streetName = streetName;}public int getHouseNumber() {return houseNumber;}public void setHouseNumber(int houseNumber) {this.houseNumber = houseNumber;}@Overridepublic int hashCode() {return getId();}@Overridepublic boolean equals(Object obj) {if (obj instanceof Address) {Address address = (Address) obj;return address.getId() == getId();}return false;}
}

我们获得了一些具有单向和双向关系的基本类。这些关系将帮助我们处理将要执行的所有类型的查询。

为了生成数据库数据，我们具有以下类：

package com.main;import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;import com.model.Address;
import com.model.Dog;
import com.model.Person;public class CodeGenerator {private static EntityManagerFactory emf;private static EntityManager em;public static final String PERSON01_NAME = 'John';public static final String PERSON02_NAME = 'Mary';public static final String PERSON03_NAME = 'Anna';public static final String PERSON04_NAME = 'Joseph';public static final String PERSON05_NAME = 'Mark';public static final String PERSON06_NAME = 'I will not have any relationship';public static void startConnection() {emf = Persistence.createEntityManagerFactory('JpaQuery');em = emf.createEntityManager();em.getTransaction().begin();}public static void closeConnection() {em.getTransaction().commit();emf.close();}public static void generateData() {int year = 1995;int month = 1;int day = 10;Dog dog01 = new Dog('Yellow', 3.5d, createNewDate(day, month, year));Dog dog02 = new Dog('Brown', 8.5d, createNewDate(++day, ++month, ++year));Dog dog03 = new Dog('Dark', 15.5d, createNewDate(++day, ++month, ++year));Dog dog04 = new Dog('Kaka', 4.3d, createNewDate(++day, ++month, ++year));Dog dog05 = new Dog('Pepe', 8.2d, createNewDate(++day, ++month, ++year));Dog dog06 = new Dog('Casillas', 6.1d, createNewDate(++day, ++month, ++year));Dog dog07 = new Dog('Fish', 6.7d, createNewDate(++day, ++month, ++year));Dog dog08 = new Dog('Lion', 3.1d, createNewDate(++day, ++month, ++year));Dog dog09 = new Dog('Cat', 5.5d, createNewDate(++day, ++month, ++year));Dog dog10 = new Dog('Java', 21.7d, createNewDate(++day, ++month, ++year));Dog dog11 = new Dog('JSF', 23.65d, createNewDate(++day, ++month, ++year));Dog dog12 = new Dog('VRaptor', 24.0d, createNewDate(++day, ++month, ++year));Dog dog13 = new Dog('Ferrari', 3.7d, createNewDate(++day, ++month, ++year));Dog dog14 = new Dog('Porshe', 1.33d, createNewDate(++day, ++month, ++year));Dog dog15 = new Dog('Bike', 4.44d, createNewDate(++day, ++month, ++year));Dog dog16 = new Dog('Rambo', 5.44d, createNewDate(++day, ++month, 2015));Dog dog17 = new Dog('Terminator', 3.88d, createNewDate(++day, ++month, 2016));Dog dog18 = new Dog('John McClan', 3.88d, createNewDate(++day, ++month, 2016));Person person01 = new Person(PERSON01_NAME, 33);person01.getDogs().add(dog01);person01.getDogs().add(dog02);person01.getDogs().add(dog03);person01.setAddress(new Address('Street A', 30));dog01.setPerson(person01);dog02.setPerson(person01);dog03.setPerson(person01);Person person02 = new Person(PERSON02_NAME, 27);person02.getDogs().add(dog04);person02.getDogs().add(dog05);person02.getDogs().add(dog06);person02.setAddress(new Address('Street B', 60));dog04.setPerson(person02);dog05.setPerson(person02);dog06.setPerson(person02);Person person03 = new Person(PERSON03_NAME, 7);person03.getDogs().add(dog07);person03.getDogs().add(dog08);person03.getDogs().add(dog09);person03.setAddress(new Address('Street B', 90));dog07.setPerson(person03);dog08.setPerson(person03);dog09.setPerson(person03);Person person04 = new Person(PERSON04_NAME, 43);person04.getDogs().add(dog10);person04.getDogs().add(dog11);person04.getDogs().add(dog12);person04.setAddress(new Address('Street C', 120));dog10.setPerson(person04);dog11.setPerson(person04);dog12.setPerson(person04);Person person05 = new Person(PERSON05_NAME, 70);person05.getDogs().add(dog13);person05.getDogs().add(dog14);person05.getDogs().add(dog15);person05.getDogs().add(dog16);person05.setAddress(new Address('Street D', 150));dog13.setPerson(person05);dog14.setPerson(person05);dog15.setPerson(person05);dog16.setPerson(person05);Person person06 = new Person(PERSON06_NAME, 45);em.persist(person01);em.persist(person02);em.persist(person03);em.persist(person04);em.persist(person05);em.persist(person06);em.persist(dog17);em.persist(dog18);em.flush();}private static Date createNewDate(int day, int month, int year) {SimpleDateFormat formatter = new SimpleDateFormat('dd/MM/yyyy');try {return formatter.parse('' + day + '/' + month + '/' + year);} catch (ParseException e) {e.printStackTrace();return null;}}public static EntityManager getEntityManager() {return em;}
}

查找方法； 使用getReference方法可获得更好的性能，并使用log4j在控制台中显示查询参数

通常在我们对数据库执行某些更改（例如更新某些对象属性，关系或删除它）之前调用find方法。

在下面，您将找到使用find方法的代码：

package com.main;import javax.persistence.EntityManager;import com.model.Address;
import com.model.Person;public class Page03 {public static void main(String[] args) {CodeGenerator.startConnection();CodeGenerator.generateData();EntityManager em = CodeGenerator.getEntityManager();Person person = em.find(Person.class, 1);int addressId = 2;// usually we send an id or a detached object from the viewsetAddressToOtherPerson(em, person, addressId);int personId = 4;// usually we send an id or a detached object from the viewdeletePerson(em, personId);CodeGenerator.closeConnection();}private static void setAddressToOtherPerson(EntityManager em, Person person, int addressId) {Address address = em.find(Address.class, addressId);person.setAddress(address);em.merge(person);em.flush();}private static void deletePerson(EntityManager em, int personId) {Person savedPerson = em.find(Person.class, personId);em.remove(savedPerson);em.flush();}
}

注意，方法“ setAddressToOtherPerson ”和“ deletePerson ”仅使用find方法来更新引用或删除对象。

find（）方法具有优化的查询功能，该功能将在持久性上下文中搜索对象，如果找不到该对象，它将查询数据库以带来数据。如果您获得了以EAGER注释的关系（例如：“ @OneToMany（fetch = FetchType.EAGER） ”），则find方法将从数据库中带走这些对象。注意，对于诸如删除参考更新之类的简单任务，无需从数据库中获取所有这些数据。

EntityManager具有帮助这些简单任务的特定方法。 EntityManager将执行一个简单的查询，例如“ 从Person p中选择id，其中p.id =：personId ”。我们将有一个更快，更小的查询。

在下面，您可以看到我们将如何使用getReference：

package com.main;import javax.persistence.EntityManager;import com.model.Address;
import com.model.Person;public class Page03 {public static void main(String[] args) {CodeGenerator.startConnection();CodeGenerator.generateData();EntityManager em = CodeGenerator.getEntityManager();Person person = em.find(Person.class, 1);int addressId = 2;// usually we send an id or a detached object from the viewsetAddressToOtherPerson(em, person, addressId);int personId = 4;// usually we send an id or a detached object from the viewdeletePerson(em, personId);CodeGenerator.closeConnection();}private static void setAddressToOtherPerson(EntityManager em, Person person, int addressId) {Address address = em.getReference(Address.class, addressId);person.setAddress(address);em.merge(person);em.flush();System.out.println('Merged');}private static void deletePerson(EntityManager em, int personId) {// usually is find or mergePerson savedPerson = em.getReference(Person.class, personId);em.remove(savedPerson);em.flush();System.out.println('Deleted');}
}

使用“ getReference ”方法，您将仅查询对象ID，将节省一些数据库流量。

在下面，您将找到在控制台中显示JPA查询参数所需的lo4j.properties配置。通常，当我们使用Hibernate调用查询时，Hibernate将使用“？”格式化查询。而不是使用实际价值。使用下面的代码，您将能够看到查询参数：

# Direct log messages to stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n# Root logger option
log4j.rootLogger=ERROR, stdout# Hibernate logging options (INFO only shows startup messages)
log4j.logger.org.hibernate=ERROR# Log JDBC bind parameter runtime arguments
log4j.logger.org.hibernate.type=TRACE

如果要停用日志，则只需在lo4j.properties的最后一行用＃符号注释，并将“ persistence.xml”中的show_log配置设置为false。

JPQL：具有简单参数或对象的查询，联接，排序依据，在关系中导航

要进行基本查询，您只需要运行如下命令：“从Dog d中选择d”。您始终需要牢记的一件事是：为了执行这种查询，我们使用JPQL而不是常规SQL 。

使用JPQL的优点是它与SQL非常相似且可移植。您可以在每个数据库中使用相同的查询而不会出现问题。

切勿将查询与字符串连接。如果您执行以下查询：“从Person p的p中选择p，其中p.name” + person.getName（），则可以确保黑客会喜欢它。他们使用这种代码进行名为“ SQL注入”（或JPQL注入）的攻击。避免这种攻击的方法是在查询中添加参数，就像下面看到的那样。

您将看到以下几种执行查询的方法：

package com.main;import java.util.List;import javax.persistence.EntityManager;
import javax.persistence.Query;import com.model.Dog;
import com.model.Person;public class Page04 {public static void main(String[] args) {CodeGenerator.startConnection();CodeGenerator.generateData();EntityManager em = CodeGenerator.getEntityManager();List<Dog> dogs = listAllDogs(em);for (Dog dog : dogs) {System.out.println(dog.getName());}Person person03 = findPersonByName(em, CodeGenerator.PERSON03_NAME);System.out.println(person03.getName());Person person01 = new Person();person01.setId(1);Person savedPerson = findPersonByPersonObject(em, person01);System.out.println(savedPerson.getName());List<Dog> dogsByWeight = listAllDogsOrderingByWeight(em);for (Dog dog : dogsByWeight) {System.out.println(dog.getWeight());}String addressName = findAddressNameOfPerson(em, CodeGenerator.PERSON04_NAME);System.out.println('Person 04 address is: ' + addressName);Person person02 = findPersonByNameWithAllDogs(em, CodeGenerator.PERSON02_NAME);for (Dog dog : person02.getDogs()) {System.out.println('Person 02 Dog: ' + dog.getName());}Person person05 = findPersonByNameThatMayNotHaveDogs(em, CodeGenerator.PERSON06_NAME);System.out.println('Is the list of the Dogs from the Person 05 empty? ' + person05.getDogs().size());CodeGenerator.closeConnection();}/*** Easiest way to do a query*/@SuppressWarnings('unchecked')private static List<Dog> listAllDogs(EntityManager em) {Query query = em.createQuery('select d from Dog d', Dog.class);return query.getResultList();}/*** Easiest way to do a query with parameters*/private static Person findPersonByName(EntityManager em, String name) {Query query = em.createQuery('select p from Person p where name = :name', Person.class);query.setParameter('name', name);return (Person) query.getSingleResult();}/*** Executes a query that has as parameter an object*/private static Person findPersonByPersonObject(EntityManager em, Person person) {Query query = em.createQuery('select p from Person p where p = :person');query.setParameter('person', person);return (Person) query.getSingleResult();}/*** Query that will list all dogs with an order*/@SuppressWarnings('unchecked')private static List<Dog> listAllDogsOrderingByWeight(EntityManager em) {Query query = em.createQuery('select d from Dog d order by d.weight desc', Dog.class);return query.getResultList();}/*** Query that get only a field instead a complete class object*/private static String findAddressNameOfPerson(EntityManager em, String name) {Query query = em.createQuery('select p.address.streetName from Person p where p.name = :name');query.setParameter('name', name);return (String) query.getSingleResult();}/*** Query that will fetch a lazy relationship Be carefull, with this kind of* query only those who have the relationship will come in the result*/private static Person findPersonByNameWithAllDogs(EntityManager em, String name) {Query query = em.createQuery('select p from Person p join fetch p.dogs where p.name = :name', Person.class);query.setParameter('name', name);return (Person) query.getSingleResult();}/*** With this query will will bring results that may not have arelationship*/private static Person findPersonByNameThatMayNotHaveDogs(EntityManager em, String name) {Query query = em.createQuery('select p from Person p left join fetch p.dogs where p.name = :name', Person.class);query.setParameter('name', name);return (Person) query.getSingleResult();}
}

关于上面的代码：

每个查询的调用方式类似于“ em.createQuery（“ HHH”，HHH.class） ”，其中包含特定的查询文本和返回类。您可以定义一个返回类，例如Person.class。 Person.class参数将向JPA指示返回对象。
我们可以使用基本属性作为查询参数，例如“ p.name =：name ”或对象“ p =：person ”。如果使用对象，则JPA将通过其@ID进行比较。
如果要订购查询，您只需要做：“ 按d.weight desc订购 ”。默认的订单值为asc，您无需编写。
关于联接，您必须注意我们使用的两种联接。在“ findPersonByNameWithAllDogs ”方法中，我们仅使用“… Person p join fetch p.dogs …”来显示狗列表。我们需要使用join fetch，因为狗列表使用“ lazy”属性进行了注释。如果我们不包括join fetch并执行了诸如“ person.getDogs（） ”之类的命令，则需要其他“ trip ”到数据库。如果使用此查询查找没有狗的人，则无论数据库是否有没有狗的人，JPA都不会在数据库中找到任何数据。如果您要执行一个查询，以获取一个获取狗的集合，而有或没有该狗的人，则需要使用“ …人p左连接获取p.dogs… ”，就像在方法中一样：“ findPersonByNameThatMayNotHaveDogs ”。 “ 左连接获取 ”将带来一个空狗列表的人。

JPQL：功能：AVG，COUNT，MAX，MIN，TRIM，SUM，UPPER，LOWER，MOD，LENGHT，SQRT； 使用HAVING，GROUP BY

JPQL还有很多功能可以帮助我们进行查询。在下面您可以看到他们的描述：

AVG –是否取平均值
COUNT –计算查询找到的记录数量
MAX –获取列的较高值
MIN-获取列的较低值
TRIM –删除文本开头/结尾处的空白
SUM –对列的所有值求和
大写–将所有列文本修改为大写
LOWER-将所有列文本修改为小写
MOD –返回列的模数
LENGTH –返回字符串的大小
SQRT –返回数字的平方根

在下面，您将看到如何使用这些功能：

package com.main;import java.util.List;import javax.persistence.EntityManager;
import javax.persistence.Query;import com.model.Person;public class Page05 {public static void main(String[] args) {CodeGenerator.startConnection();CodeGenerator.generateData();EntityManager em = CodeGenerator.getEntityManager();Number average = getPersonsAgeAverage(em);System.out.println(average);List<Object[]> personsFilteredByDogsWeight = getPersonsWithDogsWeightHigherThan(em, 4d);for (Object[] objects : personsFilteredByDogsWeight) {Person person = (Person) objects[0];Long count = (Long) objects[1];System.out.println('The person : ' + person.getName() + ' has ' + count + ' dogs with the weight > 4');}List<Object[]> dogsMinAndMaxWeightList = getDogMinAndMaxWeight(em);Object[] dogMinAndMaxWeightResult = dogsMinAndMaxWeightList.get(0);System.out.println('Min: ' + dogMinAndMaxWeightResult[0] + ' Max: ' + dogMinAndMaxWeightResult[1]);Number sumOfAllAges = getTheSumOfAllAges(em);System.out.println('All summed ages are: ' + sumOfAllAges);String loweredCaseName = getLoweredCaseNameFromUpperCase(em, CodeGenerator.PERSON03_NAME);System.out.println(loweredCaseName);Number personAgeMod = getPersonAgeMode(em, CodeGenerator.PERSON05_NAME, 6);System.out.println('Person modulus age: ' + personAgeMod);Number personAgeSqrt = getPersonAgeSqrtUsingTrim(em, '        ' + CodeGenerator.PERSON04_NAME + '        ');System.out.println('Person modulus age: ' + personAgeSqrt);List<Object[]> personsByDogsAmount = getPersonByHavingDogAmountHigherThan(em, 3);for (Object[] objects : personsByDogsAmount) {Person person = (Person) objects[0];Long count = (Long) objects[1];System.out.println(person.getName() + ' has ' + count + ' dogs');}CodeGenerator.closeConnection();}/*** Uses the AVG sql database function*/private static Number getPersonsAgeAverage(EntityManager em) {Query query = em.createQuery('select avg(p.age) from Person p');return (Number) query.getSingleResult();}/*** This query will use the count database function** @return List<Object[]> where object[0] is a person, object [2] is a Long*/@SuppressWarnings('unchecked')private static List<Object[]> getPersonsWithDogsWeightHigherThan(EntityManager em, double weight) {Query query = em.createQuery('select p, count(p) from Person p join p.dogs d where d.weight > :weight group by p');query.setParameter('weight', weight);return query.getResultList();}/*** This query will use the min and max sql database function** @return List<Object[]> where object[0] is the min, object [2] is the max*/@SuppressWarnings('unchecked')private static List<Object[]> getDogMinAndMaxWeight(EntityManager em) {Query query = em.createQuery('select min(weight), max(weight) from Dog');return query.getResultList();}/*** This query will use the sum sql database function*/private static Number getTheSumOfAllAges(EntityManager em) {Query query = em.createQuery('select sum(p.age) from Person p');return (Number) query.getSingleResult();}/*** Method that uses the UPPER and LOWER database functions*/private static String getLoweredCaseNameFromUpperCase(EntityManager em, String name) {Query query = em.createQuery('select lower(p.name) from Person p where UPPER(p.name) = :name');query.setParameter('name', name.toUpperCase());return (String) query.getSingleResult();}/*** Method that uses the mod database function*/private static Number getPersonAgeMode(EntityManager em, String personName, int modBy) {Query query = em.createQuery('select mod(p.age, :modBy) from Person p where p.name = :name');query.setParameter('modBy', modBy);query.setParameter('name', personName);return (Number) query.getSingleResult();}/*** Method that uses the square root of a person age using the trim function in the name*/private static Number getPersonAgeSqrtUsingTrim(EntityManager em, String name) {Query query = em.createQuery('select sqrt(p.age) from Person p where p.name = trim(:name)');query.setParameter('name', name);return (Number) query.getSingleResult();}/*** Method that uses the having comparator with count*/@SuppressWarnings('unchecked')private static List<Object[]> getPersonByHavingDogAmountHigherThan(EntityManager em, long dogAmount) {Query query = em.createQuery('select p, count(p) from Person p join p.dogs group by p.id having count(p) > :dogAmount');query.setParameter('dogAmount', dogAmount);return query.getResultList();}
}

关于上面的代码：

在方法“ getPersonsAgeAverage ”中，我们使用“ avg”函数对年龄列值进行平均计算。
在方法“ getPersonsWithDogsWeightHigherThan ”中，我们使用count函数将一个人对象的狗数量带入。注意，我们有两个不同的结果，一个数字和一个人物对象。这些值将位于数组Object []中。
LOWER和UPPER函数将更改您的字符串大小写，您可以使用它来更改查询结果（在选择之后）或在where条件下。 “ getLoweredCaseNameFromUpperCase ”方法以两种方式使用LOWER和UPPER函数。
“ getPersonAgeMode ”在单词select之后使用一个参数。使用JPA，我们可以在查询的任何位置使用参数，您只需要在变量中添加“：”即可。您可以多次使用相同的参数，并使用query.setParameter方法传递值。
在“ getPersonByHavingDogAmountHigherThan ”方法中，“ having ”功能与“ count ”功能一起调用。我们可以使用“ 具有 ”功能来帮助我们过滤查询数据结果。

JPQL：筛选条件：LIKE，IN，DISTINCT，EMPTY，BETWEEN，NULL，MEMBER OF，EXISTS（子查询），ANY，ALL，SOME，CONCAT，CURRENT_DATE，CURRENT_TIME，CURRENT_TIMESTAMP，LOCATE，SIZE，SUBSTRING

其中一些功能具有相同目的，但处理方式不同。

在下面您可以看到如何使用这些功能：

package com.main;import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.List;import javax.persistence.EntityManager;
import javax.persistence.Query;import com.model.Dog;
import com.model.Person;public class Page06 {public static void main(String[] args) {CodeGenerator.startConnection();CodeGenerator.generateData();EntityManager em = CodeGenerator.getEntityManager();List<Person> personByLike = getPersonByNameUsingLike(em, 'oh');for (Person person : personByLike) {System.out.println(person.getName());}List<Person> personsByAdressNumber = getPersonsByAddressNumberHigherThan(em, 90);for (Person person : personsByAdressNumber) {System.out.println(person.getName());}List<Person> personsWithoutDogs = getPersonsWithoutDogs(em);System.out.println('Total of persons without dogs: ' + personsWithoutDogs.size());List<Person> personsWithoutAddress = getPersonsWithoutAddress(em);System.out.println('Total of persons without address: ' + personsWithoutAddress.size());try {SimpleDateFormat formatter = new SimpleDateFormat('dd/MM/yyyy');Date startDate = formatter.parse('01/01/1996');Date endDate = formatter.parse('01/01/1999');List<Dog> dogsByBirth = getDogByBirthDate(em, startDate, endDate);for (Dog dog : dogsByBirth) {System.out.println(dog.getName() + ': ' + formatter.format(dog.getDateOfBirth()));}} catch (Exception e) {e.printStackTrace();}Dog dog = (Dog) em.createQuery('select d from Dog d where d.id = 1', Dog.class).getSingleResult();boolean belongsTo = isThisDogBelongingToAperson(em, dog, CodeGenerator.PERSON01_NAME);System.out.println('Is this Dog member of Perons01? ' + belongsTo);Person personByConcatedName = getPersonConcatingName(em, 'Ma', 'ry');System.out.println('Found the person? ' + personByConcatedName.getName());List<Person> personByLocate = getPersonByLocatingStringInTheName(em, 'Mary');System.out.println('Amount of persons found by locate: ' + personByLocate.size());String personNameBySubstring = getPersonNameBySubstring(em, CodeGenerator.PERSON06_NAME, 12, 18);System.out.println('Name substring is: ' + personNameBySubstring);List<Person> personsDogWeight = getPersonByDogWeightOnlyHigherThan(em, 20);for (Person person : personsDogWeight) {System.out.println(person.getName());}List<Person> distinctPersons = getDistinctPersonsByDogsWeight(em, 2d);System.out.println('With the distinct, the result size is: ' + distinctPersons.size());List<Person> personsWithDogsAmount = getPersonsWithDougsAmountOf(em, 4);System.out.println('Number of persons with 4 dogs: ' + personsWithDogsAmount.size());Number numberOfDogsByPerson = getDogAmountByPerson(em, CodeGenerator.PERSON04_NAME);System.out.println('The dog amount is to ' + CodeGenerator.PERSON04_NAME + ': ' + numberOfDogsByPerson);List<Dog> dogsBornedAfterToday = getDogsBornAfterToday(em);System.out.println('The amount of dogs borned after today is: ' + dogsBornedAfterToday.size());CodeGenerator.closeConnection();}/*** This methods compares a value with LIKE*/@SuppressWarnings('unchecked')private static List<Person> getPersonByNameUsingLike(EntityManager em, String name) {Query query = em.createQuery('select p from Person p where p.name like :name');query.setParameter('name', '%' + name + '%');return query.getResultList();}/*** This methods show several ways to do a query that checks if a part of a collection is inside another*/@SuppressWarnings('unchecked')private static List<Person> getPersonsByAddressNumberHigherThan(EntityManager em, int houseNumber) {Query query = em.createQuery('select p from Person p where p.address in (select a from Address a where a.houseNumber > :houseNumber)');// Query query = em.createQuery('select p from Person p where (select a from Address a where a.houseNumber > :houseNumber and p.address = a) > 0');// Query query = em.createQuery('select p from Person p where p.address = any (select a from Address a where a.houseNumber > :houseNumber)');// Query query = em.createQuery('select p from Person p where p.address = some (select a from Address a where a.houseNumber > :houseNumber)');// Query query = em.createQuery('select p from Person p where exists (select a from p.address a where a.houseNumber > :houseNumber)');query.setParameter('houseNumber', houseNumber);return query.getResultList();}/*** This methods show how to check if a collection is empty*/@SuppressWarnings('unchecked')private static List<Person> getPersonsWithoutDogs(EntityManager em) {Query query = em.createQuery('select p from Person p where p.dogs is empty');return query.getResultList();}/*** This method shows two ways to check if a relationship @OneToOne is empty*/@SuppressWarnings('unchecked')private static List<Person> getPersonsWithoutAddress(EntityManager em) {Query query = em.createQuery('select p from Person p where p.address is null');// Query query = em.createQuery('select p from Person p where p.address is empty');return query.getResultList();}/*** Method that uses the between comparation*/@SuppressWarnings('unchecked')private static List<Dog> getDogByBirthDate(EntityManager em, Date startDate, Date endDate) {Query query = em.createQuery('select d from Dog d where d.dateOfBirth between :startDate and :endDate');query.setParameter('startDate', startDate);query.setParameter('endDate', endDate);return query.getResultList();}/*** Method that uses the member of comparation to check if an object belogs to a collection*/private static boolean isThisDogBelongingToAperson(EntityManager em, Dog dog, String name) {Query query = em.createQuery('select p from Person p where :dog member of p.dogs and p.name = :name');query.setParameter('dog', dog);query.setParameter('name', name);try {return query.getSingleResult() != null;} catch (Exception e) {return false;}}/*** Methods that concats Strings*/private static Person getPersonConcatingName(EntityManager em, String firstWord, String secondWord) {Query query = em.createQuery('select p from Person p where p.name = concat(:firstWord, :secondWord)', Person.class);query.setParameter('firstWord', firstWord);query.setParameter('secondWord', secondWord);return (Person) query.getSingleResult();}/*** Method that locates a string inside another*/@SuppressWarnings('unchecked')private static List<Person> getPersonByLocatingStringInTheName(EntityManager em, String valueToBeLocated) {Query query = em.createQuery('select p from Person p where locate(p.name, :value) > 0', Person.class);query.setParameter('value', valueToBeLocated);return query.getResultList();}/*** Methods that uses the ALL comparator*/@SuppressWarnings('unchecked')private static List<Person> getPersonByDogWeightOnlyHigherThan(EntityManager em, double weight) {Query query = em.createQuery('select p from Person p where p.dogs is not empty and :weight < all (select d.weight from p.dogs d)');query.setParameter('weight', weight);return query.getResultList();}/*** Method that uses the distinct to remove any repetetition*/@SuppressWarnings('unchecked')private static List<Person> getDistinctPersonsByDogsWeight(EntityManager em, double weight) {Query query = em.createQuery('select distinct p from Person p join p.dogs d where d.weight > :weight');query.setParameter('weight', weight);return query.getResultList();}/*** Method that uses the substring to get just a position of chars inside the string*/private static String getPersonNameBySubstring(EntityManager em, String personName, int startPosition, int endPosition) {Query query = em.createQuery('select substring(p.name, :startPosition, :endPosition) from Person p where p.name = :personName');query.setParameter('personName', personName);query.setParameter('startPosition', startPosition);query.setParameter('endPosition', endPosition);return (String) query.getSingleResult();}/*** Method that checks the size of a collection*/@SuppressWarnings('unchecked')private static List<Person> getPersonsWithDougsAmountOf(EntityManager em, int dogAmount) {Query query = em.createQuery('select p from Person p where size(p.dogs) = :dogAmount');query.setParameter('dogAmount', dogAmount);return query.getResultList();}/*** Method that gets the size of a collection*/private static Number getDogAmountByPerson(EntityManager em, String personName) {Query query = em.createQuery('select size(p.dogs) from Person p where p.name = :personName');query.setParameter('personName', personName);return (Number) query.getSingleResult();}/*** Methods that uses the current database server date/time*/@SuppressWarnings('unchecked')private static List<Dog> getDogsBornAfterToday(EntityManager em) {Query query = em.createQuery('select d from Dog d where d.dateOfBirth > CURRENT_DATE');return query.getResultList();}
}

关于上面的代码：

您可以在查询中添加“ NOT”字样。如果使用“ IS EMPTY ”，则将搜索没有值的集合；如果您使用“ IS NOT EMPTY ”，则将搜索已填充的集合。
“ getPersonsByAddressNumberHigherThan ”显示了如何使用不同的功能执行相同的查询。所有带注释的命令行将带来相同的结果。 In / Any / Some / Exists具有紧密的语法。根据Pro EJB3的书，“ Some ”是“ Any”的别名。
比较器“ IS EMPTY ”可用于检查集合（例如@OneToMany）或关系类（例如@OneToOne）。 “ IS NULL ”比较器无法检查集合，但是您可以使用它来检查非集合属性（例如@OneToOne）。
“ MEMBER OF ”比较器将检查给定参数是否属于集合。
“ CONCAT”功能可以用作条件比较器或查询结果。在上面的代码中，它只是用作比较器，但您可以像这样使用它：“从Person p中选择concat（firstName，lastName）”
在“ getPersonByDogWeightOnlyHigherThan ”方法中，我们使用ALL运算符。仅当条件的所有项目（“ ：weight> ALL” ）都返回true时，此运算符才返回true。在这种方法中，只有当所有狗的体重都大于“：weight”时，它才会返回true；如果只有一只狗的体重值较小，则比较器将返回false。您必须知道， 如果列表为空，则比较器将返回true 。为了避免这种现象，您需要像方法中那样检查列表是否为空：“ p.dogs不为空 ”。
“ distinct ”功能将删除重复的对象。在“ getDistinctPersonsByDogsWeight ”方法中，“ distinct ”功能将删除重复的人员。
“ SUBSTRING ”函数从给定的字符串中提取一个值。您将设置将从原始值提取的值的开始和结束。您也可以将此功能用作比较器。
“ SIZE ”函数将返回集合中的元素数量。您可以用作比较器或获取值。
在上面的代码中，我们使用“ CURRENTE_DATE ”函数比较日期，您也可以使用“ CURRENT_TIME，CURRENT_TIMESTAMP ”。 JPA规范指出，当前日期函数只能用作比较器。 JPA尚不支持任何功能来检索数据库的当前日期，因为这种功能不是数据库可移植的（ 4.6.16函数表达式– JSR-000220 Enterprise JavaBeans 3.0 Final Release（持久性） ）。如果要查询数据库日期，则可以使用NativeQuery来获取此值。
我必须永远记住，您不能在集合内部导航。您不能执行以下命令：“ person.dogs.name”。您可以使用以下命令访问狗的名字： 从Person中选择p，然后取入p.dogs d，其中d.name ='' 。