JPA 子查询是 JPA 框架中的一种查询方式,它可以帮助我们在一个查询中使用另一个查询的结果。它可以让我们在一个复杂的查询中使用多个表,而不需要写出复杂的 SQL 语句。
JPA 子查询有两种形式:内连接子查询和外连接子查询。内连接子查询是将父查询和子查询关联起来,并将父查询的结果作为子查询的输入。而外连接子查询则是将父查询和子查询分开,并将子查询的结果作为父查询的输入。
// 内连接子查询 SELECT * FROM table1 t1 INNER JOIN (SELECT * FROM table2 WHERE condition) t2 ON t1.id = t2.id; // 外连JPA 查询计数其中子查询示例
JPA教程 - JPA 查询计数其中子查询示例
以下代码显示如何在子查询中使用COUNT函数。
List l = em.createQuery( "SELECT m FROM Professor m WHERE (SELECT COUNT(e) " + "FROM Professor e WHERE e.manager = m) > 0") .getResultList();例子
以下代码来自Professor.java。
package cn..common; import java.util.ArrayList; import java.util.Collection; import java.util.Date; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToMany; import javax.persistence.ManyToOne; import javax.persistence.OneToMany; import javax.persistence.OneToOne; import javax.persistence.Temporal; import javax.persistence.TemporalType; @Entity public class Professor { @Id private int id; private String name; private long salary; @Temporal(TemporalType.DATE) private Date startDate; @OneToOne private Address address; @OneToMany(mappedBy="employee") private Collection<Phone> phones = new ArrayList<Phone>(); @ManyToOne private Department department; @ManyToOne private Professor manager; @OneToMany(mappedBy="manager") private Collection<Professor> directs = new ArrayList<Professor>(); @ManyToMany(mappedBy="employees") private Collection<Project> projects = new ArrayList<Project>(); public int getId() { return id; } public void setId(int empNo) { this.id = empNo; } public String getName() { return name; } public void setName(String name) { this.name = name; } public long getSalary() { return salary; } public void setSalary(long salary) { this.salary = salary; } public Date getStartDate() { return startDate; } public void setStartDate(Date startDate) { this.startDate = startDate; } public Collection<Phone> getPhones() { return phones; } public void addPhone(Phone phone) { if (!getPhones().contains(phone)) { getPhones().add(phone); if (phone.getProfessor() != null) { phone.getProfessor().getPhones().remove(phone); } phone.setProfessor(this); } } public Department getDepartment() { return department; } public void setDepartment(Department department) { if (this.department != null) { this.department.getProfessors().remove(this); } this.department = department; this.department.getProfessors().add(this); } public Collection<Professor> getDirects() { return directs; } public void addDirect(Professor employee) { if (!getDirects().contains(employee)) { getDirects().add(employee); if (employee.getManager() != null) { employee.getManager().getDirects().remove(employee); } employee.setManager(this); } } public Professor getManager() { return manager; } public void setManager(Professor manager) { this.manager = manager; } public Collection<Project> getProjects() { return projects; } public void addProject(Project project) { if (!getProjects().contains(project)) { getProjects().add(project); } if (!project.getProfessors().contains(this)) { project.getProfessors().add(this); } } public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public String toString() { return "Professor " + getId() + ": name: " + getName() + ", salary: " + getSalary() + ", phones: " + getPhones() + ", managerNo: " + ((getManager() == null) ? null : getManager().getId()) + ", deptNo: " + ((getDepartment() == null) ? null : getDepartment().getId()); } }下面的代码来自Department.java。
package cn..common; import java.util.HashSet; import java.util.Set; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToMany; @Entity public class Department { @Id private int id; private String name; @OneToMany(mappedBy="department") private Set<Professor> employees = new HashSet<Professor>(); public int getId() { return id; } public void setId(int deptNo) { this.id = deptNo; } public String getName() { return name; } public void setName(String deptName) { this.name = deptName; } public Set<Professor> getProfessors() { return employees; } public String toString() { return "Department no: " + getId() + ", name: " + getName(); } }下面的代码来自PersonDaoImpl.java。
package cn..common; import java.util.List; import javax.persistence.EntityManager; import javax.persistence.PersistenceContext; import org.springframework.transaction.annotation.Transactional; @Transactional public class PersonDaoImpl { public void test() { prepareData(); List l = em.createQuery( "SELECT m FROM Professor m WHERE (SELECT COUNT(e) " + "FROM Professor e WHERE e.manager = m) > 0") .getResultList(); for (Object p : l) { printResult(p); } } private void prepareData() { Professor p = new Professor(); p.setId(0); p.setName("TOM"); p.setSalary(1111L); Department d = new Department(); d.setId(1); d.setName("Design"); p.setDepartment(d); d.getProfessors().add(p); em.persist(p); em.persist(d); } private static void printResult(Object result) { if (result == null) { System.out.print("NULL"); } else if (result instanceof Object[]) { Object[] row = (Object[]) result; System.out.print("["); for (int i = 0; i < row.length; i++) { printResult(row[i]); } System.out.print("]"); } else if (result instanceof Long || result instanceof Double || result instanceof String) { System.out.print(result.getClass().getName() + ": " + result); } else { System.out.print(result); } System.out.println(); } @PersistenceContext private EntityManager em; }以下代码来自Project.java。
package cn..common; import java.util.ArrayList; import java.util.Collection; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.Inheritance; import javax.persistence.ManyToMany; @Entity @Inheritance public class Project { @Id protected int id; protected String name; @ManyToMany protected Collection<Professor> employees = new ArrayList<Professor>(); public int getId() { return id; } public void setId(int projectNo) { this.id = projectNo; } public String getName() { return name; } public void setName(String projectName) { this.name = projectName; } public Collection<Professor> getProfessors() { return employees; } public void addProfessor(Professor employee) { if (!getProfessors().contains(employee)) { getProfessors().add(employee); } if (!employee.getProjects().contains(this)) { employee.getProjects().add(this); } } public String toString() { return getClass().getName().substring(getClass().getName().lastIndexOf(".")+1) + " no: " + getId() + ", name: " + getName(); } }以下代码来自Address.java。
package cn..common; import javax.persistence.Entity; import javax.persistence.Id; @Entity public class Address { @Id private int id; private String street; private String city; private String state; private String zip; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getStreet() { return street; } public void setStreet(String address) { this.street = address; } public String getCity() { return city; } public void setCity(String city) { this.city = city; } public String getState() { return state; } public void setState(String state) { this.state = state; } public String getZip() { return zip; } public void setZip(String zip) { this.zip = zip; } public String toString() { return "Address id: " + getId() + ", street: " + getStreet() + ", city: " + getCity() + ", state: " + getState() + ", zip: " + getZip(); } }以下代码来自Phone.java。
package cn..common; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToOne; @Entity public class Phone { @Id private long id; private String number; private String type; @ManyToOne Professor employee; public long getId() { return id; } public void setId(long id) { this.id = id; } public String getNumber() { return number; } public void setNumber(String phoneNo) { this.number = phoneNo; } public String getType() { return type; } public void setType(String phoneType) { this.type = phoneType; } public Professor getProfessor() { return employee; } public void setProfessor(Professor employee) { this.employee = employee; } public String toString() { return "Phone id: " + getId() + ", no: " + getNumber() + ", type: " + getType(); } }下载 Query_Count_Where_Subquery.zip以下是数据库转储。
Table Name: ADDRESS Table Name: DEPARTMENT Row: Column Name: ID, Column Type: INTEGER: Column Value: 1 Column Name: NAME, Column Type: VARCHAR: Column Value: Design Table Name: PHONE Table Name: PROFESSOR Row: Column Name: ID, Column Type: INTEGER: Column Value: 0 Column Name: NAME, Column Type: VARCHAR: Column Value: TOM Column Name: SALARY, Column Type: BIGINT: Column Value: 1111 Column Name: STARTDATE, Column Type: DATE: Column Value: null Column Name: ADDRESS_ID, Column Type: INTEGER: Column Value: null Column Name: DEPARTMENT_ID, Column Type: INTEGER: Column Value: 1 Column Name: MANAGER_ID, Column Type: INTEGER: Column Value: null Table Name: PROJECT Table Name: PROJECT_PROFESSOR
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