前言

在SQLite數據庫中，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。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

}

這里我使用了@ForeignKey關聯了company表，主鍵id，外鍵emp_id，緊接著使用了indices創建了唯一索引。

下面就是創建Dao

@Dao

public interface CompanyDao {

@Query("SELECT * FROM company")

List getAllCompany();

}

@Dao

public interface DepartmentDao {

@Query("SELECT * FROM department")

List 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 getDepartmentFromCompany();

}

最后就是創建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 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 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據插入到數據庫了，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 = 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 = 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

}

因為上面我們已經將查詢方法寫好了，直接調用即可。

List 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。后續還會繼續更新。

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找一找教程網。

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