房间嵌套查询和子查询

Question

我在我的 Android 项目中使用 Room，并且想编写一个复杂的查询。我搜索了一下，有一些答案说使用 @Embedded 像这样：

class TripAndListsAndListItems {
      @Embedded
      var trip: Trip? = null

     @Relation(parentColumn = "creatorId", entityColumn = "remoteId",     entity = User::class)
     var user: List<User>? = null

     @Relation(parentColumn = "remoteId", entityColumn = "tripId", entity = PlanitiList::class)
     var lists: List<ListAndListItems>? = null
}

Here is complete article.

但是我必须在我的代码中找出它来使用循环等提取我的结果。我在 @Query 中使用嵌套查询编写查询，并使用“as”将列与实体字段匹配，如下所示：

这里是 ViewModel class:

class ServiceCard(
    val id: Int,
    val customerInfo: String,
    val time: String,
    val oilFilter: Boolean,
    val airFilter: Boolean,
    val gasFilter: Boolean,
    val oil: Boolean
)

和@Doa有一个像这样的@Query方法：

@Dao
interface ServiceCardDao :ICommonDao<ServiceCard>{

   @Query("SELECT s.services_id as id,  " +
        "s.user_mobile_no as customerInfo, " +
        "( " +
        "SELECT count(*) " +
        "FROM service_detail as sd " +
        "WHERE sd.services_id = s.services_id and sd.service_type_id = 1 " +
        ")              as oilFilter, " +
        "( " +
        "SELECT count(*) " +
        "FROM service_detail as sd " +
        "WHERE sd.services_id = s.services_id and sd.service_type_id = 2 " +
        ")         as airFilter, " +
        "( " +
        "SELECT count(*) " +
        "FROM service_detail as sd " +
        "WHERE sd.services_id = s.services_id and sd.service_type_id = 3 " +
        ")         as gasFilter,  " +
        "( " +
        "SELECT count(*) " +
        "FROM service_detail as sd " +
        "WHERE sd.services_id = s.services_id and sd.service_type_id = 4 " +
        ")         as oil, " +
        "s.service_date as time " +
        "FROM services as s ")
   fun selectAllServicesWithDetail(): LiveData<List<model.ServiceCard>>

}

这两个有什么优缺点吗？

Answer 1

两者各有优势。

一个在Dao中的编码更复杂，另一个在Entity中的编码更复杂，但Dao中的复杂度大于Entities的复杂度差异。所以简单的道可能会得到一些人的青睐。

一个会更有效，因为没有 post 数据检索循环来获取计数，此外 SQLite 是编译的 C 代码而不是必须解释的 JVM 字节码，因此 SQlite 通常非常有效。然而，效率很可能是一些人为了更简单的编码而愿意放弃的东西，也许是为了人们习惯的东西。

有些人可能会考虑将 Dao 和 Class 合为一体的替代方案，例如 DatabaseViews。

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