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Android-Room數(shù)據(jù)庫（介紹）
前言
在SQLite數(shù)據(jù)庫中，我們可以指定對象之間的關系，因此我們可以將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，所以表之間就得有關聯(lián)。這時候我們就得使用新的注解符@ForeignKey
接下來的內容，就需要上節(jié)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(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());
}
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(lián)兩個表查詢后的字段是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
}
因為上面我們已經(jīng)將查詢方法寫好了，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());
}
查詢結果如下，說明我們多表查詢成功了：
到這里，多表查詢就結束了。后續(xù)還會繼續(xù)更新。
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持億速云。
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