Android—Room資料庫多表查詢(Relationships)

Android-Room資料庫（介紹）

Android—Room自定義型別(TypeConverter)

Android—Room 資料庫遷移(Migration)

Android — Room 資料庫跳躍式升級(Migration)

前言

在SQLite資料庫中，我們可以指定物件之間的關係，因此我們可以將一個或多個物件與一個或多個其他物件繫結。這就是所謂的一對多和多對多的關係。

既然要多表查詢，所以表之間就得有關聯。這時候我們就得使用新的註解符@ForeignKey

接下來的內容，就需要上節的內容了

@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
}

下面我們再新建一個與之關聯的表

@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
}

這裡我使用了@ForeignKey關聯了company表，主鍵id，外來鍵emp_id，緊接著使用了indices建立了唯一索引。

下面就是建立Dao

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

@Dao
public interface DepartmentDao {
    @Query("SELECT * FROM department")
    List<Department> getAllDepartment();
    //使用內連線查詢
    @Query("SELECT emp_id,name,dept  from company INNER JOIN department ON Company.id=Department.emp_id")
    List<InnerJoinResult> getDepartmentFromCompany();
}

最後就是建立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();
}

這裡我想大家經過之前的文章介紹都很熟悉了吧。這裡就不多解釋了，不記得的，請看之前的文章。

具體使用

        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);

這樣我們就把需要的資料插入到資料庫了，下面我們查詢一下，看看有沒有插入成功。

查詢程式碼如下：

     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());
      }

這樣看來是插入成功了的。。。

好了，下面我們要開始連表查詢了。

當我們查詢的時候，我們就會遇到一個問題，那就是我們關聯兩個表查詢後的欄位是company和departemnt組合之後的。所以這裡我們又需要去再次建立一個Model類。

public class InnerJoinResult {
    @ColumnInfo(name="emp_id")
    private int empId;
    private String name;
    private String dept;
    //這裡同樣省略了getter/setter方法
}

因為上面我們已經將查詢方法寫好了，直接呼叫即可。

 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());
  }

查詢結果如下，說明我們多表查詢成功了：

到這裡，多表查詢就結束了。未完待續