Android Room 的可選查詢參數

Question

我有以下帶有查詢的 DAO：

@Dao
public interface BaseballCardDao {
    @Query(
        "SELECT * FROM baseball_cards " +
        "WHERE brand LIKE :brand " +
        "  AND year = :year " +
        "  AND number LIKE :number " +
        "  AND player_name LIKE :playerName " +
        "  AND team LIKE :team"
    )
    LiveData<List<BaseballCard>> getBaseballCards(
        String brand, int year, String number, String playerName, String team
    );
}

String參數是“可選的”，因為我可以通過"%%"來匹配由於LIKE運算符的所有行。 但是我不能用year來做到這一點，因為它是一個int 。 一種解決方案是添加兩種不同的@Query方法，一種帶有int year參數，另一種沒有。 有沒有更優雅的方法來使用 Room 的@Query創建可選參數？

Answer 1

這是一個遲到的答案，但正如我最近面臨的那樣，我想與那些正在尋找它的人分享我的簡單（但很愚蠢！）技巧。

正如@CommonsWare 所說，我們可以添加一個OR語句來檢查空值，然后簡單地使我們的可選參數可以為空並為它們傳遞null 。 例如，您的查詢如下所示：

@Dao
public interface BaseballCardDao {
    @Query(
        "SELECT * FROM baseball_cards " +
        "WHERE (:brand IS NULL OR brand LIKE :brand)" +
        "  AND (:year IS NULL OR year = :year)" +
        "  AND (:number IS NULL OR number LIKE :number)" +
        "  AND (:playerName IS NULL OR player_name LIKE :playerName)" +
        "  AND (:team IS NULL OR team LIKE :team)"
    )
    LiveData<List<BaseballCard>> getBaseballCards(
        @Nullable String brand, @Nullable Integer year, @Nullable String number, @Nullable String playerName, @Nullable String team
    );
}

或者使用 kotlin 和可選參數進行更多聲明：

@Query(
    """SELECT * FROM baseball_cards 
        WHERE (:brand IS NULL OR brand LIKE :brand) 
        AND (:year IS NULL OR year = :year) 
        AND (:number IS NULL OR number LIKE :number) 
        AND (:playerName IS NULL OR player_name LIKE :playerName)
        AND (:team IS NULL OR team LIKE :team)"""
)
fun getBaseballCards(
    brand: String? = null,
    year: Int? = null,
    number: String? = null,
    playerName: String? = null,
    team: String? = null
): LiveData<List<BaseballCard>>

編輯：請考慮此解決方案對於不可為空的字段很有用。 如果該字段可以為空並且您想要查找沒有該字段值的記錄，這不是正確的查詢方式，您可以考慮動態查詢創建。