Android Room數(shù)據(jù)庫(kù)多表查詢的使用實(shí)例

更新時(shí)間：2018年03月13日 09:56:32 作者：人言落日是天涯

這篇文章主要介紹了Android Room數(shù)據(jù)庫(kù)多表查詢的使用實(shí)例，小編覺(jué)得挺不錯(cuò)的，現(xiàn)在分享給大家，也給大家做個(gè)參考。一起跟隨小編過(guò)來(lái)看看吧

前言

在SQLite數(shù)據(jù)庫(kù)中，我們可以指定對(duì)象之間的關(guān)系，因此我們可以將一個(gè)或多個(gè)對(duì)象與一個(gè)或多個(gè)其他對(duì)象綁定。這就是所謂的一對(duì)多和多對(duì)多的關(guān)系。

既然要多表查詢，所以表之間就得有關(guān)聯(lián)。這時(shí)候我們就得使用新的注解符@ForeignKey

接下來(lái)的內(nèi)容，就需要上節(jié)的內(nèi)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?cè)傩陆ㄒ粋€(gè)與之關(guān)聯(lián)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
}

這里我使用了@ForeignKey關(guān)聯(lián)了company表，主鍵id，外鍵emp_id，緊接著使用了indices創(chuàng)建了唯一索引。

下面就是創(chuàng)建Dao

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

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

最后就是創(chuàng)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();
}

這里我想大家經(jīng)過(guò)之前的文章介紹都很熟悉了吧。這里就不多解釋了，不記得的，請(qǐng)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數(shù)據(jù)插入到數(shù)據(jù)庫(kù)了，下面我們查詢一下，看看有沒(méi)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());
   }

這樣看來(lái)是插入成功了的。。。

好了，下面我們要開(kāi)始連表查詢了。

當(dāng)我們查詢的時(shí)候，我們就會(huì)遇到一個(gè)問(wèn)題，那就是我們關(guān)聯(lián)兩個(gè)表查詢后的字段是company和departemnt組合之后的。所以這里我們又需要去再次創(chuàng)建一個(gè)Model類。

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

因?yàn)樯厦嫖覀円呀?jīng)將查詢方法寫(xiě)好了，直接調(diào)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());
 }

查詢結(jié)果如下，說(shuō)明我們多表查詢成功了：