SQL optimization Case statement

Question

I believe I can optimize this sql statement by using a case statement for the Left Outer Joins.

But I have been having hard time setting up the cases, one for summing up the code types AB,CD and another for All the rest.

Appreciate any help or tips you can give me on this.

update billing set payments = isnull(bd1.amount, payments)
, payments = case 
       when payments is null then 0 
       else payments 
     end
, charges = case 
        when bd2.amount is not null then charges 
        when charges is null then 0 
        else charges 
      end
, balance = round(charges + isnull(bd1.amount, bi.payments), 2) 
from billing bi 

left outer join (select inv, round(sum(bd1.bal), 2) amount 
                from "bill" bd1 
                where code_type = 'AB'
                 or code_type = 'CD' 
                group by inv) bd1 
                on bd1.inv = bi.inv 
left outer join (select invoice, round(sum(bd2.bal), 2) amount 
                from "bill" bd2 
                where code_type <> 'AB'
                 and code_type <> 'CD' 
                group by inv) bd2 
                on bd2.inv = bi.inv;

Answer 1

You can simplify it to this to use a single query rather than two. You still need the one because a GROUP BY in an UPDATE doesn't work.

UPDATE bi 
SET    payments = bd.payments, 
       charges= bd.charges, 
       balance = bd.balance 
FROM   billing bi 
       LEFT JOIN (SELECT bd.inv, 
                         payments = Round(Sum(CASE 
                                                WHEN code_type IN ( 'AB' , 'CD' ) THEN 
                                                bd.bal 
                                                ELSE 0 
                                              END), 2), 
                         charges = Round(Sum(CASE 
                                               WHEN code_type NOT IN ( 'AB' , 'CD' ) THEN 
                                               bd.bal 
                                               ELSE 0 
                                             END), 2), 
                         balance = Round(Sum(bd.bal), 2) 
                  FROM   bill bd 
                  GROUP  BY bd.inv) bd 
         ON bd.inv = bi.inv 

Answer 2

Maybe something like this:

update billing set payments = isnull(bd1.amount, payments)
, payments = isnull(payments, 0)
, charges = isnull(bd2.amount, isnull(charges, 0))
, balance = round(charges + isnull(bd1.amount, bi.payments), 2)
from billing bi 

left outer join (select inv, round(sum(bd1.bal), 2) amount 
                from "bill" bd1 
                where code_type in ('AB', 'CD')
                group by inv) bd1 
                on bd1.inv = bi.inv 
left outer join (select invoice, round(sum(bd2.bal), 2) amount 
                from "bill" bd2 
                where code_type not in ('AB', 'CD')
                group by inv) bd2 
                on bd2.inv = bi.inv;

The two left joins are not a problem!