The Prolog below reproduces the situation described above. The two sets of Prolog "facts": suppliers and supplies are the same as the "proper" way to do database management in the write-up above. These are the Prolog analog to "normalized" relations in database terms. The code below illustrates that one can easily construct the "bad" relation sup_info from the two that are provided. The Prolog code also illustrates how to print information. (In the code below, Prolog tries to match up the information in a rule. The the final presense of fail in the rule tells Prolog to try to match a different way. Because there are write in these rules, a side-effect of the matching is to print the data.)

In the listing below, boldface black is user input, blue is the Prolog source, and red is the output from the Prolog program. (In the final listing, I added few extra tabs by hand.)

[wagner@gateway19 db_design]$ cat supply2.pl
/* SUPPLIERS (SNAME, SADDR) */
suppliers(sunshine_produce, addr_16_river_st).
suppliers(purity_foodstuffs, addr_180_industrial_rd).
suppliers(tasti_supply_co, addr_17_river_st).
/* SUPPLIES (SNAME, ITEM, PRICE) */
supplies(sunshine_produce, granola, 129).
supplies(sunshine_produce, lettuce, 89).
supplies(sunshine_produce, sunflower_seeds, 109).
supplies(purity_foodstuffs, whey, 70).
supplies(purity_foodstuffs, curds, 80).
supplies(purity_foodstuffs, granola, 125).
supplies(purity_foodstuffs, unbleached_flour, 65).
supplies(tasti_supply_co, lettuce, 79).
supplies(tasti_supply_co, whey, 79).
supplies(tasti_supply_co, sunflower_seeds, 119).

items(ITEM, SNAME, SADDR) :-
   supplies(SNAME, ITEM, _), suppliers(SNAME, SADDR).
   
items_print(ITEM) :-
   items(ITEM, SNAME, SADDR), 
   write(ITEM), write('  '),
   write(SNAME), write('  '),
   write(SADDR), write('  '), nl, fail.
   
sup_info(SNAME, SADDR, ITEM, PRICE) :-
   suppliers(SNAME, SADDR), supplies(SNAME, ITEM, PRICE).
   
sup_info_print(_) :-
   sup_info(SNAME, SADDR, ITEM, PRICE),
   write(SNAME), write(' \t'),
   write(SADDR), write(' \t'),
   write(ITEM), write(' \t'),
   write(PRICE), nl, fail.
[wagner@gateway19 db_design]$ pl
Welcome to SWI-Prolog (Multi-threaded, Version 5.2.13)
Copyright (c) 1990-2003 University of Amsterdam.
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use ?- help(Topic). or ?- apropos(Word).

?- consult(supply2).
% supply2 compiled 0.00 sec, 3,504 bytes

Yes
?- items(granola, SNAME, SADDR).

SNAME = sunshine_produce
SADDR = addr_16_river_st ;

SNAME = purity_foodstuffs
SADDR = addr_180_industrial_rd ;

No
?- items_print(granola).
granola  sunshine_produce  addr_16_river_st  
granola  purity_foodstuffs  addr_180_industrial_rd  

No
?- sup_info(X, Y, Z, W).

X = sunshine_produce
Y = addr_16_river_st
Z = granola
W = 129 ;

X = sunshine_produce
Y = addr_16_river_st
Z = lettuce
W = 89 ;
 
  (many lines deleted)

X = tasti_supply_co
Y = addr_17_river_st
Z = sunflower_seeds
W = 119 ;

No
?- sup_info_print(_).
sunshine_produce        addr_16_river_st        granola                 129
sunshine_produce        addr_16_river_st        lettuce                 89
sunshine_produce        addr_16_river_st        sunflower_seeds         109
purity_foodstuffs       addr_180_industrial_rd  whey                    70
purity_foodstuffs       addr_180_industrial_rd  curds                   80
purity_foodstuffs       addr_180_industrial_rd  granola                 125
purity_foodstuffs       addr_180_industrial_rd  unbleached_flour        65
tasti_supply_co         addr_17_river_st        lettuce                 79
tasti_supply_co         addr_17_river_st        whey                    79
tasti_supply_co         addr_17_river_st        sunflower_seeds         119

No
?- halt.
[wagner@gateway19 db_design]$