Android 房间：针对字符串列查询列表项

Question

我有一个字符串列表：

val mylist = listOf("cat","flower")

和一个 table，它有一个名为 question 的字符串类型的列我可以编写查询来查找与列表项之一完全匹配的问题：

@Query("SELECT * FROM objects WHERE question IN (:mylist)")
List<Object> queryObjects(List<String> mylist);

但实际上问题列数据不是单词类型，而是字符串。我需要找到每个列表项都在该字符串中的结果。例如记录：is this a cat

Answer 1

IN 的使用基本上是对 IN 子句左侧的表达式与右侧值列表的 = 测试。那只是考虑完全匹配。

然而，你想要的是多个带有通配符的 LIKE，并且每个 LIKE 之间有一个 OR 例如 question LIKE '%cat%' OR question LIKE '%flower%' 或 CASE WHEN THEN ELSE END 或者递归通用 table 表达式 (CTE)。

前两个（LIKE 或 CASE）可能必须通过 @RawQuery 完成，其中 LIKE/CASE 子句是在运行时构建的。

递归 CTE 选项基本上会构建一个单词列表（但如果包含标点符号等除空格以外的任何内容，可能会变得更加复杂。）

另一种选择是考虑全文搜索 (FTS)。不妨参考https://www.raywenderlich.com/14292824-full-text-search-in-room-tutorial-getting-started

工作示例 赞

下面是一个最简单的例子，多个 LIKE 子句用 OR 分隔：-

对象（实体）：-

@Entity
data class Objects(
    @PrimaryKey
    val id: Long? = null,
    val question: String
)

AllDAO（道）：-

@Dao
interface AllDAO {

    @Insert(onConflict = OnConflictStrategy.IGNORE)
    fun insert(objects: Objects)

    @RawQuery
    fun getObjectsRawQuery(query: SupportSQLiteQuery): List<Objects>

    fun getObjects(values: List<String>): List<Objects> {
        var i = 0
        val sb = StringBuilder().append("SELECT * FROM objects WHERE ")
        for(v in values) {
            if (i++ > 0) {
                sb.append(" OR ")
            }
            sb.append(" question LIKE '%${v}%'")
        }
        sb.append(";")
        return getObjectsRawQuery(SimpleSQLiteQuery(sb.toString()))
    }
}

TheDatabase（为方便起见，未使用 .allowMainThreadQueries）：-

@Database(entities = [Objects::class], version = 1, exportSchema = false)
abstract class TheDatabase: RoomDatabase() {
    abstract fun getAllDAO(): AllDAO

    companion object {
        var instance: TheDatabase? = null
        fun getInstance(context: Context): TheDatabase {
            if (instance == null) {
                instance = Room.databaseBuilder(context,TheDatabase::class.java,"the_database.db")
                    .allowMainThreadQueries()
                    .build()
            }
            return instance as TheDatabase
        }
    }
}

将它们放在一起，加载一些测试数据并运行提取一些数据：-

class MainActivity : AppCompatActivity() {
    lateinit var db: TheDatabase
    lateinit var dao: AllDAO
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        db = TheDatabase.getInstance(this)
        dao = db.getAllDAO()

        dao.insert(Objects(question = "This is a cat."))
        dao.insert(Objects(question = "This is a flower."))
        dao.insert(Objects(question = "this is nothing."))
        dao.insert(Objects(question = "The quick brown fox jumped over the lazy dog"))

        logObjects(dao.getObjects(listOf("cat","dog")),"Extract1\t")
        logObjects(dao.getObjects(listOf("flower","cat")),"Extract2\t")
        logObjects(dao.getObjects(listOf("brown","nothing")),"Extract3\t")
    }

    fun logObjects(objects: List<Objects>,prefix: String) {
        for (o in objects) {
            Log.d("OBJECTINFO","$prefix Question is ${o.question} ID is ${o.id}")
        }
    }
}

结果

2022-04-18 04:58:05.471 D/OBJECTINFO: Extract1   Question is This is a cat. ID is 1
2022-04-18 04:58:05.471 D/OBJECTINFO: Extract1   Question is The quick brown fox jumped over the lazy dog ID is 4

2022-04-18 04:58:05.473 D/OBJECTINFO: Extract2   Question is This is a cat. ID is 1
2022-04-18 04:58:05.473 D/OBJECTINFO: Extract2   Question is This is a flower. ID is 2

2022-04-18 04:58:05.474 D/OBJECTINFO: Extract3   Question is this is nothing. ID is 3
2022-04-18 04:58:05.474 D/OBJECTINFO: Extract3   Question is The quick brown fox jumped over the lazy dog ID is 4

注意上面没有考虑处理空列表（由于SELECT * FROM objects WHERE ;的语法错误会导致失败）。也就是说这个例子只是为了演示基本原理。

Android 房间：针对字符串列查询列表项

Android room: query list items against string column

android

kotlin

android-room