题解 | #商品交易(网易校招笔试真题)#

```WITH t AS (
	SELECT
		b.char_len,
	CASE
			
			WHEN a.author_level IN ( 1, 2 ) THEN
			"1-2级" 
			WHEN a.author_level IN ( 3, 4 ) THEN
			"3-4级" ELSE "5-6级" 
		END AS level_cut 
	FROM
		author_tb a
		LEFT JOIN answer_tb b ON a.author_id = b.author_id 
	WHERE
		char_len >= 100 
	) SELECT
	level_cut,
	count( char_len ) num 
FROM
	t 
GROUP BY
	level_cut 
ORDER BY
	num DESC
解题思路：
  根据连接键连接两表为一表，添加一个标记字段重新划分级别。 过滤掉回答低于100字符的记录，剩下的回答记录都是高质量回答。接着按照标记字段分组，统计高质量回答数即可。