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());