What does it mean SELECT EXISTS (SELECT 1 FROM favoritelist WHERE id=:id)

What is the meaning of SELECT EXISTS (SELECT 1 FROM favoritelist WHERE id=:id) in:

@Query("SELECT EXISTS (SELECT 1 FROM favoritelist WHERE id=:id)")
public int  isFavorite(int id);

Answer

SELECT EXISTS (SELECT 1 FROM favoritelist WHERE id=:id)

The exists condition evaluates whether at least one row exists in favoritelist where id matches the given parameter. The outer query evaluates the condition in numeric context, and returns 1 if the condition is satisfied, else 0.

In a nutshell:

• if at least one record exists in favoritelist whose id matches the parameter, the query returns 1

• else, it returns 0