Question

I have two tables

customer_master
> cname
> lastreceiptdate
> lastreceiptamt

accounts
> cname
> date
> amount

I need help in constructing a single update query. where the customer_master table is updated with the latest receipt date and receipt amount for a single customer code (cname like “FRUITXXXXX”) from accounts table.

It should work in mysql 5.0 and postgresql 9.0

thanks

so far we are using a select command to retrieve a record with max(Date) and then using another update command to update using results from the select query.

update customer_master 
set receiptdate = @_da, receiptamt=@_amt 
where cname = @_cn and (receiptdate < @_da or receiptdate='null') 

Answer 1

If you need to do this for all customers, it will be a lot faster to truncate the table and insert everything in one go:

truncate table customer_master;
insert into customer_master (cname, lastreceiptdate, lastreceiptamt)
select cname, last_date, last_amount
from amounts
  join (
    select cname, 
           max(date) as max_date
    from accounts
    group by cname
  ) ma on ma.max_date = amounts.date 
      and ma.cname = amounts.cname

This asumes that the max. date is “unique” i.e. that not two rows with the same date exists in accounts.

If you really, really want to update the table each time something changes, you can use something like this:

update customer_master
    set lastreceiptdate = last_date, 
        lastreceiptamt = last_amount
from (
  select cname, last_date, last_amount
  from amounts
    join (
      select cname, 
             max(date) as max_date
      from accounts
      group by cname
    ) ma on ma.max_date = amounts.date 
        and ma.cname = amounts.cname
) as t
where customer_master.cname = t.cname
and customer_master.cname = 'some customer name';

The customer_master.cname = 'some customer name' runs the update for a single customer, not for all. (I don’t know if that is going to work in MySQL though)

Note that you do need to JOIN on the group by statement in order to get the correct amount that “belongs” to the latest receiptdate.

If you use a simple

select cname,
       max(date), 
       max(amount)
from accounts;

the returned max. amount does not necessarily “belong” to the max. date (it could be an amount from a different date).