Integrity constraint for tables not immediately related - sql

Question

I am a SQL beginner and I can't figure out how to properly create an integrity constraint for situations like this:
The schema describes a delivery system - each Restaurant offers some items, which can be delivered to customers (outside the visible schema).
The problem comes with the in_delivery table - items from menu are registered with the delivery through this table. With the current state of things, it is possible to add a menu_item to a delivery which is done by a restaurant, but that restaurant may not offer the menu_item!
When inserting into in_delivery, I need to somehow check, if the Menu_Item_MenuItem_ID is present in offers, that has Restaurant_RestaurantID equal to RestaurantID in Delivery associated with the table.
I don't know if I can use a foreign key here, because the tables are not "adjacent"..
What comes into mind is to have a RestaurantID in in_delivery, that would be a foreign key both to Restaurant and Delivery. Then I could find that in offers. Is there a better way?
Thanks for your help

Answer 1

You could enforce your constraint with the following changes:
add the restaurant_id column in the in_delivery table
add a unique constraint on delivery (delivery_id, restaurant_id) (needed for 3.)
change the foreign key from in_delivery -> delivery to point to (delivery_id, restaurant_id)
change the foreign key from in_delivery -> menu_item to in_delivery -> offers

Answer 2

Alternatively you can use a trigger to check the constraint:
SQL Fiddle
Oracle 11g R2 Schema Setup:
CREATE TABLE Restaurants (
RestaurantID NUMBER(2) PRIMARY KEY,
Name VARCHAR2(30) NOT NULL
)
/
INSERT INTO Restaurants
SELECT 1, 'Soylent Green Express' FROM DUAL
UNION ALL SELECT 2, 'Helga''s House of Ribs' FROM DUAL
/
CREATE TABLE Menu_Items (
Menu_Item_ID NUMBER(2) PRIMARY KEY,
Name VARCHAR2(20) NOT NULL
)
/
INSERT INTO Menu_Items
SELECT 1, 'Soylent Green' FROM DUAL
UNION ALL SELECT 2, 'Ribs' FROM DUAL
/
CREATE TABLE Offers (
RestaurantID NUMBER(2),
Menu_Item_ID NUMBER(2),
PRIMARY KEY ( RestaurantID, Menu_Item_ID ),
FOREIGN KEY ( RestaurantID ) REFERENCES Restaurants ( RestaurantID ),
FOREIGN KEY ( Menu_Item_ID ) REFERENCES Menu_Items ( Menu_Item_ID )
)
/
INSERT INTO Offers
SELECT 1, 1 FROM DUAL
UNION ALL SELECT 2, 2 FROM DUAL
/
CREATE TABLE Deliveries (
RestaurantID NUMBER(2) NOT NULL,
Delivery_ID NUMBER(2) PRIMARY KEY,
FOREIGN KEY ( RestaurantID ) REFERENCES Restaurants ( RestaurantID )
)
/
INSERT INTO Deliveries
SELECT 1, 1 FROM DUAL
UNION ALL SELECT 2, 2 FROM DUAL
/
CREATE TABLE in_delivery (
Delivery_ID NUMBER(2),
Menu_Item_ID NUMBER(2),
PRIMARY KEY ( Delivery_ID, Menu_Item_ID ),
FOREIGN KEY ( Delivery_ID ) REFERENCES Deliveries ( Delivery_ID ),
FOREIGN KEY ( Menu_Item_ID ) REFERENCES Menu_Items ( Menu_Item_ID )
)
/
Just for ease of reading I've created two useful functions (you would probably want some exception handling in them):
CREATE OR REPLACE FUNCTION get_Delivery_RestaurantID (
p_Delivery_ID Deliveries.Delivery_ID%TYPE
) RETURN Restaurants.RestaurantID%TYPE
AS
v_RestaurantID Restaurants.RestaurantID%TYPE;
BEGIN
SELECT RestaurantID
INTO v_RestaurantID
FROM Deliveries
WHERE Delivery_ID = p_Delivery_ID;
RETURN v_RestaurantID;
END get_Delivery_RestaurantID;
/
CREATE OR REPLACE FUNCTION does_Restaurant_Offer_Item (
p_RestaurantID Restaurants.RestaurantID%TYPE,
p_Menu_Item_ID Menu_Items.Menu_Item_ID%TYPE
) RETURN NUMBER
AS
v_exists NUMBER(1);
BEGIN
SELECT CASE WHEN EXISTS ( SELECT 1
FROM Offers
WHERE RestaurantID = p_RestaurantID
AND Menu_Item_ID = p_Menu_Item_ID
)
THEN 1
ELSE 0
END
INTO v_exists
FROM DUAL;
RETURN v_exists;
END does_Restaurant_Offer_Item;
/
Then just add a trigger to the table to check that the Restaurant offers the item and, if not, raise an exception.
CREATE TRIGGER check_Valid_Delivery_Item
BEFORE INSERT OR UPDATE OF Delivery_ID, Menu_Item_ID
ON in_delivery
FOR EACH ROW
BEGIN
IF does_restaurant_Offer_Item( get_Delivery_RestaurantID( :new.Delivery_ID ), :new.Menu_Item_ID ) = 0
THEN
RAISE_APPLICATION_ERROR (-20100, 'Invalid Delivery Item');
END IF;
END check_Valid_Delivery_Item;
/
INSERT INTO in_delivery VALUES( 1, 1 )
/
INSERT INTO in_delivery VALUES( 2, 2 )
/
Query 1:
SELECT * FROM in_delivery
Results:
| DELIVERY_ID | MENU_ITEM_ID |
|-------------|--------------|
| 1 | 1 |
| 2 | 2 |
If you try to do:
INSERT INTO in_delivery VALUES( 1, 2 );
Then you get:
ORA-20100: Invalid Delivery Item ORA-06512: at "USER_4_F9593.CHECK_VALID_DELIVERY_ITEM", line 4 ORA-04088: error during execution of trigger 'USER_4_F9593.CHECK_VALID_DELIVERY_ITEM' : INSERT INTO in_delivery VALUES( 1, 2 )