在实际过往的项目中，常用的查询操作有：1、单表查询，2、一对一查询（主表和详情表）3、一对多查询（一张主表，多张子表）4、多对多查询（如权限控制，用户、角色多对多）。做个总结，所以废话不多说。

　　使用idea构建springboot项目，引入依赖如下：

dependencies>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-data-jpa</artifactId>

        </dependency>

        <dependency>

            <groupId>com.h2database</groupId>

            <artifactId>h2</artifactId>

            <scope>runtime</scope>

        </dependency>

        <dependency>

            <groupId>org.projectlombok</groupId>

            <artifactId>lombok</artifactId>

            <optional>true</optional>

        </dependency>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-test</artifactId>

            <scope>test</scope>

        </dependency>

    </dependencies>

　　使用h2数据库做测试用，application.yml配置如下：

spring:

  jpa:

    generate-ddl: true

    hibernate:

      ddl-auto: update

    properties:

      hibenate:

        format_sql: false

    show-sql: true

　　首先，一对一有好几种，这里举例的是常用的一对一双向外键关联（改造成单向很简单，在对应的实体类去掉要关联其它实体的属性即可），并且配置了级联删除和添加，相关类如下：

package io.powerx;

import lombok.*;

import javax.persistence.*;

/**

 * Created by Administrator on 2018/8/15.

 */

@Getter

@Setter

@Entity

public class Book {

    @Id

    @GeneratedValue

    private Integer id;

    private String name;

    @OneToOne(cascade = {CascadeType.PERSIST,CascadeType.REMOVE})

    @JoinColumn(name="detailId",referencedColumnName = "id")

    private BookDetail bookDetail;

    public Book(){

        super();

    }

    public Book(String name){

        super();

        this.name =name;

    }

    public Book(String name, BookDetail bookDetail) {

        super();

        this.name = name;

        this.bookDetail = bookDetail;

    }

    @Override

    public String toString() {

        if (null == bookDetail) {

            return String.format("Book [id=%s, name=%s, number of pages=%s]", id, name, "<EMPTY>");

        }

        return String.format("Book [id=%s, name=%s, number of pages=%s]", id, name, bookDetail.getNumberOfPages());

    }

}
package io.powerx;

import lombok.Getter;

import lombok.Setter;

import javax.persistence.*;

@Getter

@Setter

@Entity(name = "BOOK_DETAIL")

public class BookDetail {

    @Id

    @GeneratedValue

    private Integer id;

    @Column(name = "NUMBER_OF_PAGES")

    private Integer numberOfPages;

    @OneToOne(mappedBy = "bookDetail")

    private Book book;

    public BookDetail() {

        super();

    }

    public BookDetail(Integer numberOfPages) {

        super();

        this.numberOfPages = numberOfPages;

    }

    @Override

    public String toString() {

        if (null == book) {

            return String.format("Book [id=%s, name=%s, number of pages=%s]", id, "<EMPTY>");

        }

        return String.format("Book [id=%s, name=%s, number of pages=%s]", id,book.getId(),book.getName());

    }

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**

 * Created by Administrator on 2018/8/15.

 */

public interface BookRepository extends JpaRepository<Book,Integer> {

    Book findByName(String name);

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**

 * Created by Administrator on 2018/8/15.

 */

public interface BookDetailRepository  extends JpaRepository<BookDetail, Integer>{

    BookDetail findByNumberOfPages(Integer numberOfPages);

}
package io.powerx;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.test.context.SpringBootTest;

import org.springframework.test.context.junit4.SpringRunner;

import java.util.Arrays;

import static org.junit.Assert.assertThat;

@RunWith(SpringRunner.class)

@SpringBootTest

public class OnetooneApplicationTests {

    @Autowired

    private BookRepository bookRepository;

    @Autowired

    private BookDetailRepository bookDetailRepository;

    @Before

    public void init() {

        Book bookA = new Book("Spring in Action", new BookDetail(208));

        Book bookB = new Book("Spring Data in Action", new BookDetail(235));

        Book bookC = new Book("Spring Boot in Action");

        bookRepository.saveAll(Arrays.asList(bookA, bookB, bookC));

    }

    @After

    public void clear() {

        bookRepository.deleteAll();

    }

    @Test

    public void find() {

        Book book = bookRepository.findByName("Spring in Action");

        System.err.println(book.toString());

    }

    @Test

    public void save() {

        Book book = new Book("springboot");

        BookDetail bookDetail = new BookDetail(124);

        book.setBookDetail(bookDetail);

        bookRepository.save(book);

    }

    @Test

    public void delete() {

        bookRepository.deleteById(31);

    }

    @Test

    public void findbook(){

        BookDetail bd = bookDetailRepository.findByNumberOfPages(235);

        System.err.println(bd.toString());

    }

}

　　一对多双向，相关类如下：

package io.powerx;

import lombok.Data;

import lombok.Getter;

import lombok.Setter;

import javax.persistence.*;

@Getter

@Setter

@Entity

public class Book {

    @Id

    @GeneratedValue

    private Integer id;

    private String name;

    @ManyToOne

    @JoinColumn(name="publishId")

    private Publisher publisher;

    @Override

    public String toString() {

        return "Book{" +

                "id=" + id +

                ", name='" + name + '\'' +

                ", publisher=" + publisher.getName() +

                '}';

    }

    public Book(String name) {

        this.name = name;

    }

    public Book() {

    }

}
package io.powerx;

import lombok.Data;

import lombok.Getter;

import lombok.Setter;

import javax.persistence.*;

import java.util.HashSet;

import java.util.Set;

/**

 * Created by Administrator on 2018/8/16.

 */

@Getter

@Setter

@Entity

public class Publisher {

    @Id

    @GeneratedValue

    private Integer id;

    private String name;

    @OneToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)

    @JoinColumn(name="publishId",referencedColumnName = "id")

    private Set<Book> books;

    public Publisher() {

        super();

    }

    public Publisher(String name) {

        super();

        this.name = name;

    }

    @Override

    public String toString() {

        return "Publisher{" +

                "id=" + id +

                ", name='" + name + '\'' +

                ", books=" + books.size() +

                '}';

    }

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**

 * Created by Administrator on 2018/8/16.

 */

public interface BookRepository  extends JpaRepository<Book,Integer>{

    Book findByName(String name);

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

/**

 * Created by Administrator on 2018/8/16.

 */

public interface PublisherRepository extends JpaRepository<Publisher,Integer> {

    Publisher findByName(String name);

}
package io.powerx;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.test.context.SpringBootTest;

import org.springframework.test.context.junit4.SpringRunner;

import java.util.HashSet;

import java.util.Set;

@RunWith(SpringRunner.class)

@SpringBootTest

public class OnetomanyApplicationTests {

    @Autowired

    private PublisherRepository publisherRepository;

    @Autowired

    private BookRepository bookRepository;

    @Before

    public void init() {

        Book book1 = new Book("spring");

        Book book2 = new Book("mvc");

        Book book3 = new Book("mybatis");

        Publisher publisher = new Publisher("zhonghua");

        Set<Book> set = new HashSet<Book>();

        set.add(book1);

        set.add(book2);

        set.add(book3);

        publisher.setBooks(set);

        publisherRepository.save(publisher);

    }

    @After

    public void clear() {

        publisherRepository.deleteAll();

    }

    @Test

    public void find() {

        Publisher publisher = publisherRepository.findByName("zhonghua");

        System.out.println(publisher);

    }

    @Test

    public void find2() {

        Book book = bookRepository.findByName("mvc");

        System.out.println(book);

    }

}

　　多对多双向，相关代码如下：

package io.powerx;

import lombok.Getter;

import lombok.Setter;

import lombok.ToString;

import javax.persistence.*;

import java.util.Set;

@Getter

@Setter

@Entity

public class Author {

    @Id

    @GeneratedValue

    private Integer id;

    private String name;

    @ManyToMany(mappedBy = "authors",fetch = FetchType.EAGER)

    private Set<Book> books;

    public Author() {

        super();

    }

    public Author(String name) {

        super();

        this.name = name;

    }

    @Override

    public String toString() {

        return "Author{" +

                "id=" + id +

                ", name='" + name + '\'' +

                ", books=" + books.size() +

                '}';

    }

}
package io.powerx;

import lombok.Getter;

import lombok.Setter;

import lombok.ToString;

import javax.persistence.*;

import java.util.HashSet;

import java.util.Set;

@Getter

@Setter

@Entity

public class Book {

    @Id

    @GeneratedValue

    private Integer id;

    private String name;

    @ManyToMany(cascade = CascadeType.ALL,fetch = FetchType.EAGER)

    @JoinTable(name = "BOOK_AUTHOR", joinColumns = {

            @JoinColumn(name = "BOOK_ID", referencedColumnName = "ID")}, inverseJoinColumns = {

            @JoinColumn(name = "AUTHOR_ID", referencedColumnName = "ID")})

    private Set<Author> authors;

    public Book() {

        super();

    }

    public Book(String name) {

        super();

        this.name = name;

        this.authors = new HashSet<>();

    }

    public Book(String name, Set<Author> authors) {

        super();

        this.name = name;

        this.authors = authors;

    }

    @Override

    public String toString() {

        return "Book{" +

                "id=" + id +

                ", name='" + name + '\'' +

                ", authors=" + authors.size() +

                '}';

    }

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface AuthorRepository extends JpaRepository<Author, Integer> {

    Author findByName(String name);

    List<Author> findByNameContaining(String name);

}
package io.powerx;

import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

public interface BookRepository extends JpaRepository<Book, Integer> {

    Book findByName(String name);

    List<Book> findByNameContaining(String name);

}

　　在调试过程中，注意实体类的tostring方法的重写，避免相互引用；此外如果超过两张表的关联查询，建议使用自定义sql，建立相应的pojo来接收查询结果。

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