Conditional SQL I/O

If an RPG programmer has a best friend, it may very well be the CHAIN opcode, which performs a random read on a keyed database file. It’s common programming practice in information-processing shops to read one file sequentially and to chain to read a host of others randomly. SQL handles this sort of thing differently. With SQL, joining is the proper way to retrieve data from two or more tables and/or views. The difference is even more pronounced when some of the random processing may or may not take place.

Suppose you’re responsible for a database that contains customer orders. Each order is for one item, and the price of an item comes from one of two tables: a standard pricing table (the catalog prices), and a customer-specific pricing table. The usual way to retrieve the price is to look for (randomly read) a customer-specific price first, and if none is found, then and only then look for (randomly read) the standard price. In RPG terms, it’s like this:

dow '1';
   read order;
   if %eof();
      leave;
   endif;
   // look for a price for this customer & item
   chain (custid: itemid) cusprice;
   if %found();
      SellingPrice = Price;
   else;
      // there was no price for this customer
      // get the catalog price
      chain itemid PriceRec;
      if %found();
         SellingPrice = Price;
      else;
         // there is no price for item
         // perform error routine
      endif;
   endif;
enddo;

But SQL contains no such conditional logic to control I/O. In an SQL world, the way to handle such a situation is to read everything, whether you need it or not. This is counterintuitive to an old RPG or COBOL programmer, but it works, and it works just fine.

Take a look at this SQL command:

select ord.ordernbr, ord.custid, ord.itemid,
       cp.price, prc.price,
       coalesce(cp.price, prc.price) as SellingPrice
  from orders as ord
  left join prices as prc
    on ord.itemid = prc.itemid
  left join cusprices as cp
    on ord.custid = cp.custid
   and ord.itemid = cp.itemid

The primary table, ORDERS, is the one that would typically be read sequentially in an RPG or COBOL program. PRICES (the standard price table) and CUSPRICES (the customer price table) are both included as secondary tables. The conditional logic–giving preference to the customer price over the standard price–is carried out with the COALESCE function.

COALESCE takes two or more arguments and returns the first one that is not null. In this case, if the customer price (cp.price) is not null, the customer price is the selling price. Otherwise, the standard price (prc.price) is the selling price. The result set illustrates this behavior.

select ord.ordernbr, ord.custid, cus.custgrp,
       ord.itemid, cp.price, gp.price, prc.price,
       coalesce(cp.price, gp.price, prc.price) as UnitPrice
  from orders as ord
  left join prices as prc
    on ord.itemid = prc.itemid
  left join cusprices as cp
    on ord.custid = cp.custid
   and ord.itemid = cp.itemid
  left join customers as cus
    on ord.custid = cus.custid
  left join cusprices as gp
    on cus.cusgrp = gp.custid