如何根据另一个表的记录数更新SQL表？

Question

I have two SQL tables named as Share and Balance . users is common column in both table. There are multiple rows/share for each user in Share table.

What I need to do is update paid column from Share table by adding 1 on each share and that added count no needs to be added in balance column of Balance table. Say, a user has 3 rows/shares in Share table and if i add +1 for each share then in balance column of Balance table needs to add 3 with existing balance.

I have written following sql statement. It is updating Share table accordingly but Balance table is updating wrong. See the image below:

Update Share Set paid=paid + 1
Update Balance Set balance=balance + S.Paid
From balance B,share S Where B.users=S.users

Answer 1

Update Share Set paid=paid + 1
Update Balance Set balance=balance + (select count(S.Paid) From balance B,share S 
                                     Where B.users=S.users
                                     group by s.users )
From balance B,share S Where B.users=S.users

--If increment is always +1 in share table then this query will work fine.

Answer 2

You could use two select.

For readblity is better the explicit join sintax
and if you use alias you should use the table alias in all the columns name

Update Share 
Set paid=paid + 1;

Update Balance B
INNER JOIN (
  select users, count(*) an num_count
  from share
  group by user
  ) t on t.users = B.users
Set B.balance=B.balance + t.num_count;