1) ER Diagram for Online Food Order System:

Entities: Customer, Restaurant, Order, Food Item, Delivery, Payment, Review
Relationships:
Customer places Order (one-to-many)
Order contains Food Item (many-to-many)
Order is delivered by Delivery (one-to-one)
Payment is related to Order (one-to-one)
Customer writes Review for Restaurant (one-to-many)

2) PL/SQL Procedure to Calculate Square of a Number:

CREATE OR REPLACE PROCEDURE calculate_square(
    num IN NUMBER,
    square OUT NUMBER
) AS
BEGIN
    square := num * num;
END;
3) SQL Queries:
A) SELECT supplier_number, supplier_name FROM Suppliers WHERE supplier_name LIKE 'R%';
B)
SELECT supplier_name 
FROM Suppliers 
WHERE item_supplied = 'Processor' AND city = 'Delhi';
C)
SELECT DISTINCT S.supplier_name 
FROM Suppliers S 
JOIN Suppliers R ON S.item_supplied = R.item_supplied 
WHERE R.supplier_name = 'Ramesh';
D) UPDATE Items SET item_price = item_price + 200 WHERE item_name = 'Keyboard';
E)
SELECT supplier_number, supplier_name, item_price 
FROM Suppliers 
WHERE city = 'Delhi' 
ORDER BY item_price ASC;
F) ALTER TABLE Suppliers ADD contact_no VARCHAR2(20);
G)
DELETE FROM Items 
WHERE item_price = (SELECT MIN(item_price) FROM Items);
H)
CREATE VIEW SupplierDetails AS
SELECT supplier_number, supplier_name 
FROM Suppliers;