Question

我具有实体类Person，并且我想通过feature从数据库中查询人，该人可以为null，也可以不为null。当我将功能设置为非null时，查询有效，但当功能为null时，查询返回空列表。我在做什么错了？

测试

Context context = InstrumentationRegistry.getContext();
AppDb db = Room.inMemoryDatabaseBuilder(context, AppDb.class).allowMainThreadQueries().build();
PersonDao dao = db.personDao();

dao.insert(new Person("El Risitas", "Funny"));
dao.insert(new Person("Elon Musk", "Alien"));
dao.insert(new Person("Donald Trump", null));

assertEquals(3, dao.getAll().size());

assertEquals("Funny", dao.getByFeature("Funny").get(0).getFeature());

// fails because dao.getByFeature(null) = EMPTY LIST
assertEquals(null, dao.getByFeature(null).get(0).getFeature());

Person.java

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName = "person")
public class Person {

    @ColumnInfo(name = "id") @PrimaryKey(autoGenerate = true) private int id;
    @ColumnInfo(name = "name") private String name;
    @ColumnInfo(name = "feature") private String feature;

    public Person(String name, String feature) {
        this.name = name;
        this.feature = feature;
    }
    public void setId(int id) { this.id = id; }
    public int getId() { return id; }
    public String getName() { return name; }
    public String getFeature() { return feature; }
}

PersonDao.java

import androidx.room.Dao;
import androidx.room.Insert;
import androidx.room.Query;
import java.util.List;

@Dao
public interface PersonDao {

    @Insert
    void insert(Person person);

    @Query("SELECT * FROM person")
    List<Person> getAll();

    @Query("SELECT * FROM person WHERE feature = :feature")
    List<Person> getByFeature(String feature);

}

AppDb.java

import androidx.room.Database;
import androidx.room.RoomDatabase;

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

    public abstract PersonDao personDao();
}

Answer 1

在SQL中，没有什么等于空。（不都不等于null。）您必须使用is null。因此您的查询可能类似于（未经测试）

SELECT * FROM person WHERE feature = :feature or (feature is null and :feature is null)

带有空参数的Room（SQLite）WHERE子句不起作用

1 个答案: