3764. Most Common Course Pairs

Table: course_completions +-------------------+---------+ | Column Name | Type | +-------------------+---------+ | user_id | int | | course_id | int | | course_name | varchar | | completion_date | date | | course_rating | int | +-------------------+---------+ (user_id, course_id) is the combination of columns with unique values for this table. Each row represents a completed course by a user with