Android Room資料庫多表查詢的使用執行個體

Android-Room資料庫（介紹）

前言

在SQLite資料庫中，我們可以指定對象之間的關系，是以我們可以将一個或多個對象與一個或多個其他對象綁定。這就是所謂的一對多和多對多的關系。

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

接下來的内容，就需要上節的内容了

@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
}           

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下面我們再建立一個與之關聯的表

@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
}           

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這裡我使用了@ForeignKey關聯了company表，主鍵id，外鍵emp_id，緊接着使用了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();
}           

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

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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();
}           

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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);           

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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。

查詢代碼如下：

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());
   }           

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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字段是company和departemnt組合之後的。是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
}           

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因為上面我們已經将查詢方法寫好了，直接調用即可。

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());
 }           

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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。後續還會繼續更新。

以上就是本文的全部内容，希望對大家的學習有所幫助。