房间嵌套查询和子查询

Question

我在Android项目中使用Room，并希望编写一个复杂的查询。我搜索了一下，女巫说了一些答案，像这样使用@ Embedded：

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.

但是我必须在代码中弄清楚，才能使用循环等提取结果。我在@ Query中使用嵌套查询编写了查询，并通过使用“ as”将具有实体字段的列匹配，如下所示：

这里是ViewModel类：

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具有这样的@ 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>>

}

这两个之间有什么优点或缺点？

Answer 1

两者都有优势。

Dao中的编码对于一个实体而言更为复杂，而另一种实体中的编码则更为复杂，但是Dao中的复杂性大于实体的复杂性差异。因此，较简单的Dao可能会赢得某些人的青睐。

一种效率更高，因为没有后期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。但是，效率很可能是一些人希望为了更简单的编码而放弃的，或者可能是人们习惯使用的。]

有些人可能会考虑将DatabaseViews之类的替代方案，它将Dao和Class结合为一体。