题解 | #月总刷题数和日均刷题数#

```SELECT
	exam_id,
	SUBSTRING_INDEX( tag, ',', 1 ) tag,
	SUBSTRING_INDEX( SUBSTRING_INDEX( tag, ',', 2 ), ',',- 1 ) difficulty,
	SUBSTRING_INDEX( tag, ',',- 1 ) duration 
FROM
	examination_info 
WHERE
	id in ( SELECT id FROM examination_info WHERE difficulty = 0 AND duration = 0 )