Jetpack Room - Room 查询返回列表无需判空、LIKE 关键字

Room 查询返回列表无需判空

1、基本介绍

2、演示

（1）Application

• MyApplication.java

public class MyApplication extends Application {

private static Context context;

@Override
public void onCreate() {
super.onCreate();

context = this;
}

public static Context getContext() {
return context;
}
}

（2）Entity

• Member.java

@Entity(tableName = "Member")
public class Member {

@PrimaryKey(autoGenerate = true)
private Integer id;

@ColumnInfo(name = "name")
private String name;

@ColumnInfo(name = "age")
private Integer age;

public Member() {
}

@Ignore
public Member(String name, Integer age) {
this.name = name;
this.age = age;
}

@Ignore
public Member(Integer id, String name, Integer age) {
this.id = id;
this.name = name;
this.age = age;
}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Integer getAge() {
return age;
}

public void setAge(Integer age) {
this.age = age;
}

@Override
public String toString() {
return "Member{" +
"id=" + id +
", name='" + name + '\\'' +
", age=" + age +
'}';
}
}

（3）Dao

• MemberDAO.java

@Dao
public interface MemberDAO {

@Insert
void insert(Member member);

@Query("DELETE FROM Member")
void deleteAll();

@Query("SELECT * FROM Member")
List<Member> queryAll();

@Query("SELECT * FROM Member WHERE name = :name")
Member queryByName(String name);

@Query("SELECT * FROM Member WHERE name LIKE '%' || :name || '%'")
List<Member> queryByNameLike(String name);
}

（4）Database

• MyDatabase.java

@Database(entities = {Member.class}, version = 1, exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {

public static final String DATABASE_NAME = "test.db";

private static MyDatabase myDatabase;

public static MyDatabase getInstance() {
if (myDatabase == null) {
myDatabase = Room.databaseBuilder(MyApplication.getContext(), MyDatabase.class, DATABASE_NAME).build();
}
return myDatabase;
}

public abstract MemberDAO getMemberDAO();
}

（5）Test

new Thread(() -> {
MemberDAO memberDAO = MyDatabase.getInstance().getMemberDAO();

memberDAO.deleteAll();

Log.i(TAG, "没有数据时");

List<Member> members = memberDAO.queryAll();
Member member = memberDAO.queryByName("jack");
List<Member> membersLike = memberDAO.queryByNameLike("m");

Log.d(TAG, "members: " + members);
Log.d(TAG, "member: " + member);
Log.d(TAG, "membersLike: " + membersLike);

memberDAO.insert(new Member("Jack", 18));
memberDAO.insert(new Member("Tom", 19));
memberDAO.insert(new Member("Smith", 20));

Log.i(TAG, "有数据时");

List<Member> members2 = memberDAO.queryAll();
Member member2 = memberDAO.queryByName("Jack");
List<Member> membersLike2 = memberDAO.queryByNameLike("m");

Log.d(TAG, "members: " + members2);
Log.d(TAG, "member: " + member2);
Log.d(TAG, "membersLike: " + membersLike2);
}).start();

# 输出结果

没有数据时
members: []
member: null
membersLike: []
有数据时
members: [Member{id=4, name='Jack', age=18}, Member{id=5, name='Tom', age=19}, Member{id=6, name='Smith', age=20}]
member: Member{id=4, name='Jack', age=18}
membersLike: [Member{id=5, name='Tom', age=19}, Member{id=6, name='Smith', age=20}]

LIKE 关键字

1、基本介绍

• SQLite 中的 LIKE 关键字用于模式匹配，通常与 WHERE 子句结合使用

2、基本使用

— 查找名字以 '张' 开头的记录

SELECT * FROM users WHERE name LIKE '张%';

— 查找邮箱包含 'gmail' 的记录

SELECT * FROM users WHERE email LIKE '%gmail%';

— 查找名字第二个字符是 'a' 的记录

SELECT * FROM users WHERE name LIKE '_a%';

// Room 中的使用

@Query("SELECT * FROM Member WHERE name LIKE '%' || :name || '%'")
List<Member> queryByNameLike(String name);