题解 | #话题的分布情况#

select
    substring_index (subject_set, ',', 1) subject_id1,
    count(*) cnt
from
    comment_detail
where
    substring_index ((substring_index (subject_set, ',', 2)), ',', -1) = '1002'
group by
    subject_id1
order by
    subject_id1

题目不难，就是出题人说的不明白。这个题目想要查出第二个话题是'1002'对应的每一行数据的第一个话题的数目，很明显要

group by，然后再排序，这道题目考察的substring_index函数算是少见的函数。这篇文章讲substring_index讲的挺好

https://blog.51cto.com/u_15162069/2749344