Mysql count duplicate value different column

问题

I want to count the same value among parent_id and id_article, but it can be 0 if there is no same value among parent_id and id_article

table:t_article
id_article      parent_id
441             0
1093            18
18              0
3141            3130
3130            0
3140            3130
3142            3130

Expected output

id_article      parent_id       Total
441             0               0
1093            18              0
18              0               1
3141            3130            0
3130            0               3
3140            3130            0
3142            3130            0

How do I make it happen?

回答1:

You can get your count by doing a sub clause and then join with your main query

select a.*, coalesce(b.cnt,0)
from t_article a
left join (
  select parent_id, sum(parent_id <> 0) cnt
  from t_article 
  group by parent_id
) b on (a.id_article = b.parent_id)

Demo

来源：https://stackoverflow.com/questions/50715267/mysql-count-duplicate-value-different-column