Consider the following schema:

EMPLOYEE (SSN, Name)
CUSTOMER (CustID, CustName, Address, City, State)
ORDER (OrderID, SSN, CustID, OrderDesc, Amount)

1 a. Write relational algebra expressions for each of the following. 
     i) The SSN of all employees.
    ii) The names of all customers.
   iii) The names of all customers in Chicago. 
    iv) The customer IDs of customers that have not placed orders.
     v) The description and amount of orders made by Citibank for more than 
        $50,000. 
    vi) The name and address of customers that have placed orders for more 
        than $75,000.

1 b. The following are relational algebra statements. Examine each statement 
     carefully. Determine the tuples of the resulting relation (see instances
     of each relation below). 
     i) EMPLOYEE [SSN] + ORDER [SSN]
    ii) EMPLOYEE [SSN] - ORDER [SSN]
   iii) EMPLOYEE [SSN] X ORDER [OrderID]

2. Relational algebra is said to be closed. Briefly describe this notion.

3. Briefly describe union compatibility. Provide examples to illustrate your 
   answer. 

 

EMPLOYEE (SSN, Name)

SSN              Name
-------------------------------
145-09-966 John Brown
145-09-967 Mary Williams


CUSTOMER (CustID, CustName, Address, City, State)

CustID  CustName  Address          City    State
------------------------------------------------
000008  Citibank  243 S Dearborn   Chicago   IL
000009  AON       300 S Wabash     Chicago   IL
000010  Comdisco  200 River Road   Rosemont  IL
000011  Sprint    110 Riverdale    Austin    TX      


ORDER (OrderID, SSN, CustID, OrderDesc, Amount)

OrderID  SSN         CustID  OrderDesc        Amount
----------------------------------------------------
   100   145-09-966  000008  Pens            5000.00
   102   145-09-968  000008  Pencils        55000.00
   103   145-09-966  000011  Notepads       25000.00