Question

i have USERS TABLE that contains the following fields _id name number_of_posts

and i have POSTS TABLE that contains the following fields _id user_id post_text

no the relationship between the users and posts is one- to -many i.e one user have many post

the question is how to update the number_of_posts in users table such that it will hold the number of post in the posts table

UPDATE

I have these tables filled with about hundreds of records there is no way to use the trigger to update the number_of_posts .

Answer 1

You can do this with a trigger on the posts table. Every time a post is inserted, update the number_of_posts on the users table.

See http://dev.mysql.com/doc/refman/5.6/en/triggers.html

Update: full worked solution

drop table if exists users;
create table users (  
   _id bigint unsigned auto_increment primary key,   
   name varchar(50) not null, 
   number_of_posts integer not null default 0
);

drop table if exists posts;
create table posts (  
   _id bigint unsigned auto_increment primary key,   
   user_id bigint unsigned not null, 
   post_text text
);

-- Populate with dummy data
insert into users (name) values ('Bob'), ('Sally');
insert into posts (user_id, post_text) 
    values (1, 'First!!'), (1, 'Second...'), 
           (2, 'Post 1'), (2, 'another post'), (2, 'more posts');

-- One-time update of users table
update users u
set u.number_of_posts = (      
  select count(0) from posts p
  where u._id = p.user_id      
);

-- trigger to keep post count up to date when future posts are made
drop trigger if exists trg_post_count;
delimiter $$

create trigger trg_post_count
after insert on posts
for each row 
begin
    select count(0) into @post_count 
    from posts
    where user_id = NEW.user_id;

    update users
    set number_of_posts = @post_count
    where _id = NEW.user_id;
end;
$$

delimiter ;

-- test trigger
insert into posts (user_id) values (2);
select * from users;