In this lesson, we'll describe the @Query annotation and see how it can be used to read data from the Room database.

We'l see how Room's compile-time query checking works to ensure that the SQL you write is valid from the moment you write it.

We demonstrate how to write a function in the DAO which returns all of the Tasks currently in the database using the @Query annotation.

For this lesson, we manually call the DAO to obtain tasks from the onCreate method and after we've inserted a Task into the DB. This pattern of manually calling the refresh function isn't ideal and we'll go on to highlight a better way of doing this using LiveData in another lesson. But for now, this will teach the basics of how to invoke a query defined using the @Query annotation.

Summary of Content:

• android.arch.persistence.room.Query Annotation
• Room provides compile-time query checking for @Query
• Room provides syntax highlighting on the query
• Not just checking syntax; Room actually checks the query against the database schema you have defined
• Room also checks the return value specified to ensure what is being returned from the SQL query can be mapped to the specified return type
• How to read the DB from a background thread but update the UI from the main thread

Links:

• Android Training - Room
• @Query annotation