Re: olympics ranking query

[Kyle Bateman <kyle(at)actarg(dot)com>]

Several years ago someone posted a question about how to achieve a 
running total of columns in sql.  I have been trying to find a solution 
to a similar problem for some time and just came up with something that 
works great for me so I thought I'd post it for the benefit of the list.

The problem is to produce a query like the following:

select date,amount,run_sum(amount) from ledger order by date;

Date        Amount     Sum
----------- ---------- ---------
2007-Jan-01      10.00     10.00
2007-Jan-02      20.00     30.00
2007-Jan-05       5.00     35.00
2007-Jan-10      -3.00     32.00
.
.
.

Using pl/tcl, I made the following function:

#Keep a running sum on a column where tag and trans are used to keep the
#results distinct from other calls to the same function
#Usage: run_sum(value,tag,trans)
#-------------------------------------------
function run_sum(numeric,text,text) {} {
 returns numeric language pltcl called on null input as $$
   if {![info exists GD(sum.$2.$3)]} {
       set GD(sum.$2.$3) 0.00
   }
   if {[argisnull 1]} {
       return $GD(sum.$2.$3)
   } else {
       return [set GD(sum.$2.$3) [expr $GD(sum.$2.$3) + $1]]
   }
$$;}

Then I added a wrapper function to automatically produce a separate 
namespace for each statement instance that uses the query:

#Keep a running sum on a column where tag is used to keep the results 
distinct
#Automatically keeps results distinct for each separate statement
#Usage: run_sum(value,tag)
#-------------------------------------------
function run_sum(numeric,text) {run_sum(numeric,text,text)} {
 returns numeric language sql as $$
   select run_sum($1,$2,statement_timestamp()::text);
$$;}

Now you can do:

select date,amount,run_sum(amount,'amount') from ledger;

to get an initial result.  The only problem is now ordering the data.  
If you just put an 'order by' clause on the end, you don't get what you 
might expect because the ordering happens after the function has 
produced its result.  So I do the following to order and sum it correctly:

select date,amount,run_sum(amount,'amount') from (select * from ledger 
order by date) ss;

The use of the "tag" parameter allows you to use this on multiple 
columns such as:

select date,debit,credit,
   run_sum(debit,'debit')::numeric(14,2) as debits,
   run_sum(credit,'credit')::numeric(14,2) as credits
   from (select * from ledger order by date) ss;

Enjoy,

Kyle