pancake_constraints.sql

CREATE TABLE pancakes (
  /* The order number may be NULL - the kitchen has to eat, too! */
  order_number integer UNIQUE,
  
  /* Gotta have syrup */
  maple_syrup_brand text NOT NULL,
  
  /* Require order number and maple syrup to be unique */
  CONSTRAINT order_syrup UNIQUE (order_number, maple_syrup_brand)
 );
 
 /* Regular customer orders */
 INSERT INTO pancakes VALUES (1, 'O Canada');
 INSERT INTO pancakes VALUES (2, 'Pure Vermont Maple Syrup');
 INSERT INTO pancakes VALUES (3, 'O Canada');
 
/* Chef makes himself a snack. Notice that this doesn't trigger the constraint,
   because NULLs are treated as unique. */
INSERT INTO pancakes VALUES (NULL, 'O Canada');
INSERT INTO pancakes VALUES (NULL, 'O Canada');

/* Whoops! We already made that order! */
INSERT INTO pancakes VALUES (2, 'Pure Vermont Maple Syrup');