3421. Find Students Who Improved

Table: Scores +-------------+---------+ | Column Name | Type | +-------------+---------+ | student_id | int | | subject | varchar | | score | int | | exam_date | varchar | +-------------+---------+ (student_id, subject, exam_date) is the primary key for this table. Each row contains information about a student's score in a specific subject on a particular exam date. score is between 0 and 100 (in