Android Room 的可选查询参数

Question

我有以下带有查询的 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 参数是“可选的”，因为LIKE 运算符我可以传递"%%" 来匹配所有行。但我不能用year 做到这一点，因为它是int。一种解决方案是添加两种不同的@Query 方法，一种带有int year 参数，另一种没有。有没有更优雅的方法来使用 Room 的 @Query 创建可选参数？

Answer 1

这是一个迟到的答案，但正如我最近遇到的那样，我想与那些正在寻找它的人分享我的简单（但愚蠢！）技巧。

正如@CommonsWare 所说，我们可以添加一个OR 语句来检查它是否为空，然后简单地使我们的可选参数可以为空并为它们传递null。 例如，您的查询如下所示：

@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 和可选参数进行更多声明：

@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>>

编辑： 请考虑此解决方案对于不可为空的字段很有用。如果该字段可以为空，并且您想查找该字段没有值的记录，这不是正确的查询方式，您可以考虑创建动态查询。