mysql - How to update one table based on the count value of another table while matching ids? -

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there 2 tables: articles , comments. there association between them: each comment holds article id belongs to.

-----------------------------  articles ----+-------+----------------  id | title | comments_count ----+-------+----------------  1  |  aaa  |              0  2  |  bbb  |              0  3  |  ccc  |              0  4  |  ddd  |              0  5  |  eee  |              0  6  |  fff  |              0

...

-------------------------------  comments ----+-------------+------------  id |        text | article_id ----+-------------+------------  1  | aaa comment |          1  2  | aaa comment |          1  3  | fff comment |          6  4  | bbb comment |          2  5  | ddd comment |          4  6  | bbb comment |          2

the articles table further has columns holds current count of associated comments. here sqlfiddle basic setup of described tables.

after having imported bunch of articles , comments how can update comments_count on every article reflect number of associated comments?

try query:

update articles left join (  select   article_id,   count(*) count  comments  group article_id ) b on a.id = b.article_id set comments_count = coalesce(b.count, 0);

i used on sqlfiddle (had put in schema section) selecting articles showed correct counts me.

updated set correct value (you can run multiple times , after deletes, etc)


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