前言

在SQLite数据库中，我们可以指定对象之间的关系，因此我们可以将一个或多个对象与一个或多个其他对象绑定。这就是所谓的一对多和多对多的关系。

既然要多表查询，所以表之间就得有关联。这时候我们就得使用新的注解符@ForeignKey

接下来的内容，就需要上节的内容了

@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

}

下面我们再新建一个与之关联的表

@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

}

这里我使用了@ForeignKey关联了company表，主键id，外键emp_id，紧接着使用了indices创建了唯一索引。

下面就是创建Dao

@Dao

public interface CompanyDao {

@Query("SELECT * FROM company")

List getAllCompany();

}

@Dao

public interface DepartmentDao {

@Query("SELECT * FROM department")

List getAllDepartment();

//使用内连接查询

@Query("SELECT emp_id,name,dept from company INNER JOIN department ON Company.id=Department.emp_id")

List getDepartmentFromCompany();

}

最后就是创建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();

}

这里我想大家经过之前的文章介绍都很熟悉了吧。这里就不多解释了，不记得的，请看之前的文章。

具体使用

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);

这样我们就把需要的数据插入到数据库了，下面我们查询一下，看看有没有插入成功。

查询代码如下：

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());

}

这样看来是插入成功了的。。。

好了，下面我们要开始连表查询了。

当我们查询的时候，我们就会遇到一个问题，那就是我们关联两个表查询后的字段是company和departemnt组合之后的。所以这里我们又需要去再次创建一个Model类。

public class InnerJoinResult {

@ColumnInfo(name="emp_id")

private int empId;

private String name;

private String dept;

//这里同样省略了getter/setter方法

}

因为上面我们已经将查询方法写好了，直接调用即可。

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());

}

查询结果如下，说明我们多表查询成功了：

到这里，多表查询就结束了。后续还会继续更新。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持找一找教程网。