Android Room数据库多表查询的使用实例

Android-Room数据库（介绍）

前言

在SQLite数据库中，我们可以指定对象之间的关系，因此我们可以将一个或多个对象与一个或多个其他对象绑定。这就是所谓的一对多和多对多的关系。

既然要多表查询，所以表之间就得有关联。这时候我们就得使用新的注解符@ForeignKey

接下来的内容，就需要上节的内容了

@Entity
public class Company {
  @PrimaryKey(autoGenerate = true)
  private int id;
  private String name;
  private int age;
  private String address;
  private double salary;

  public Company(String name, int age, String address, double salary) {
    this.name = name;
    this.age = age;
    this.address = address;
    this.salary = salary;
  }
  //省略了getter/setter方法
}

下面我们再新建一个与之关联的表

@Entity(foreignKeys = @ForeignKey(entity = Company.class,parentColumns = "id",childColumns = "emp_id",onDelete = CASCADE),
    indices = @Index(value={"emp_id"},unique = true))
public class Department {
  @PrimaryKey(autoGenerate = true)
  private int id;
  private String dept;
  @ColumnInfo(name = "emp_id")
  private int empId;

  public Department(String dept, int empId) {
    this.dept = dept;
    this.empId = empId;
  }
  //省略了getter/setter方法
}

这里我使用了@ForeignKey关联了company表，主键id，外键emp_id，紧接着使用了indices创建了唯一索引。

下面就是创建Dao

@Dao
public interface CompanyDao {
  @Query("SELECT * FROM company")
  List<Company> getAllCompany();
}

@Dao
public interface DepartmentDao {
  @Query("SELECT * FROM department")
  List<Department> getAllDepartment();
  //使用内连接查询
  @Query("SELECT emp_id,name,dept from company INNER JOIN department ON Company.id=Department.emp_id")
  List<InnerJoinResult> getDepartmentFromCompany();
}

最后就是创建Database

@Database(entities = {Department.class, Company.class}, version = 1, exportSchema = false)
public abstract class DepartmentDatabase extends RoomDatabase {
  public static final String DB_NAME = "CompanyDatabase.db";
  private static volatile DepartmentDatabase instance;

  public static synchronized DepartmentDatabase getInstance(Context context) {
    if (instance == null) {
      instance = create(context);
    }
    return instance;
  }

  private static DepartmentDatabase create(final Context context) {
    return Room.databaseBuilder(
        context,
        DepartmentDatabase.class,
        DB_NAME).allowMainThreadQueries().build();
  }

  public abstract DepartmentDao getDepartmentDao();

  public abstract CompanyDao getCompanyDao();
}

这里我想大家经过之前的文章介绍都很熟悉了吧。这里就不多解释了，不记得的，请看之前的文章。

具体使用

    List<Company> list = new ArrayList<>();
    Company company = new Company("Paul",32,"California",20000.0);
    list.add(company);
    company = new Company("Allen",25,"Texas",15000.0);
    list.add(company);
    company = new Company("Teddy",23,"Norway",20000.0);
    list.add(company);
    company = new Company("Mark",25,"Rich-Mond",65000.0);
    list.add(company);
    company = new Company("David",27,"Texas",85000.0);
    list.add(company);
    company = new Company("Kim",22,"South-Hall",45000.0);
    list.add(company);
    company = new Company("James",24,"Houston",10000.0);
    list.add(company);

    List<Department> departmentList = new ArrayList<>();
    Department department = new Department("IT Billing",1);
    departmentList.add(department);
    department = new Department("Engineerin",2);
    departmentList.add(department);
    department = new Department("Finance",7);
    departmentList.add(department);

    DepartmentDatabase.getInstance(this)
    .getCompanyDao().insert(list);

    DepartmentDatabase.getInstance(this)
        .getDepartmentDao().insert(departmentList);

这样我们就把需要的数据插入到数据库了，下面我们查询一下，看看有没有插入成功。

查询代码如下：

   List<Company> company = DepartmentDatabase.getInstance(this).getCompanyDao().getAllCompany();
   LogUtil.debug("Company----->" + company.size());
   for (Company result : company) {
      LogUtil.debug("result--->" + result.getName() + " " + result.getAge()+" "+result.getAddress()+" "+result.getSalary());
   }
   List<Department> department = DepartmentDatabase.getInstance(this).getDepartmentDao().getAllDepartment();
   LogUtil.debug("Department----->" + department.size());
   for (Department result : department) {
       LogUtil.debug("result--->" + result.getDept() + " " + result.getEmpId());
   }

这样看来是插入成功了的。。。

好了，下面我们要开始连表查询了。

当我们查询的时候，我们就会遇到一个问题，那就是我们关联两个表查询后的字段是company和departemnt组合之后的。所以这里我们又需要去再次创建一个Model类。

public class InnerJoinResult {
  @ColumnInfo(name="emp_id")
  private int empId;
  private String name;
  private String dept;
  //这里同样省略了getter/setter方法
}

因为上面我们已经将查询方法写好了，直接调用即可。

 List<InnerJoinResult> company1 = DepartmentDatabase.getInstance(this).getDepartmentDao().getDepartmentFromCompany();
 LogUtil.debug("InnerJoinResult----->" + company1.size());
 for (InnerJoinResult result : company1) {
    LogUtil.debug("result--->" + result.getEmpId() + " " + result.getName() + " " + result.getDept());
 }

查询结果如下，说明我们多表查询成功了：

到这里，多表查询就结束了。后续还会继续更新。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持我们。

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