Room DAO Order By ASC or DESC variable

问题

I'm trying to make a @Query function in my @Dao interface which has a boolean parameter, isAsc to determine the order:

@Query("SELECT * FROM Persons ORDER BY first_name (:isAsc ? ASC : DESC)")
List<Person> getPersonsAlphabetically(boolean isAsc);

Apparently this isn't allowed. Is there a work around here?

EDIT:

It seemed odd to use two queries (below) since the only difference is ASC and DESC:

@Query("SELECT * FROM Persons ORDER BY last_name ASC")
List<Person> getPersonsSortByAscLastName();

@Query("SELECT * FROM Persons ORDER BY last_name DESC")
List<Person> getPersonsSortByDescLastName();

回答1:

Use CASE Expression for SQLite to achieve this in Room DAO,

@Query("SELECT * FROM Persons ORDER BY 
        CASE WHEN :isAsc = 1 THEN first_name END ASC, 
        CASE WHEN :isAsc = 0 THEN first_name END DESC")
List<Person> getPersonsAlphabetically(boolean isAsc);

回答2:

Create two queries, one with ASC and one with DESC.

@Query("SELECT * FROM Persons ORDER BY last_name ASC")
List<Person> getPersonsSortByAscLastName();

@Query("SELECT * FROM Persons ORDER BY last_name DESC")
List<Person> getPersonsSortByDescLastName();

回答3:

Why don't you try something like this? I have not tested it.

@Query("SELECT * FROM Persons ORDER BY first_name :order")
List<Person> getPersonsAlphabetically(String order);

And the logic you suggested should go before you make the query.

来源：https://stackoverflow.com/questions/55297165/room-dao-order-by-asc-or-desc-variable