Question

I have a table called users where the employee data are stored.
Also I have another table payment_details where employee’s payment related data are stored
the two tables are as follows.

this table is users

sr_no  emp_no  username  payment

1     1001      leroy    <null>
2     1003      harry    <null>
3     1004      Tom      <null>   
4     1008      Jon      <null>    

This table below is payment_details

    sr_no       name     number      month    status        date
      43        Jon       1008       January  paid         5/16/2012
      44        Jon       1008       January  balance      5/16/2012
      45        Harry     1003       January  paid         5/16/2012
      46        Tom       1004       January  paid         5/16/2012
      47        leroy     1001       January  paid         5/16/2012
      48        Jon       1008       January  paid         5/16/2012
      49        Harry     1003       January  paid         5/16/2012
      50        Jon       1008       February balance      5/16/2012
      51        leroy     1001       February paid         5/16/2012
      52        Jon       1008       February paid         5/16/2012
      53        Tom       1004       February balance      5/16/2012

My question here is to update “users” table payment column to “paid” when the status of his/her is all paid in payment_details table

Answer 1

You can either do this: http://www.sqlfiddle.com/#!3/db13f/18

update users set payment = 'paid'
from
(
  select number
  from payment_details
  group by number
  having sum(case when status = 'paid' then 1 end) 
       = count(*)
) as x
where x.number = users.emp_no;

Or this: http://www.sqlfiddle.com/#!3/db13f/19

update users
  set payment = x.upd
from 
(
  select u.emp_no, 

      case when sum(case when d.status = 'paid' then 1 end)  = count(*) then
         'paid'
      else
         null
      end as upd

  from users u

  left join payment_details d
  on d.number = u.emp_no

  group by u.emp_no
) as x
where x.emp_no = users.emp_no;

Their difference is how many rows it updates. On the second query, it updates all users, regardless if the user has a paid all status('paid') or not(null); on the first query, it updates only those who are paid.

The advantage of the second query, is when you change one of the all 'paid' status on payment_detail of a given user to ‘not paid’ for example, it can revert back the user’s payment status to null