quick refrehers

Question 1

what happens when join condition is ommited in sql

Answer 1

Cartesian product

SELECT FNAME, SSN
FROM EMPLOYEE, DEPARTMENT
WHERE DNAME=‘Research’ ( And Dno = Dnumber is ommited);
→ CARTESIAN PRODUCT (between subset of DEPARTMENT and everything of EMPLOYEE)
which can result in very large relations

Question 2

EXISTS → checks if the inner query contains tuples (isn’t empty)
NOT EXISTS does the opposite

• Example :
  SELECT *
  FROM EMPLOYEE
  WHERE EXISTS (SELECT *
  FROM DEPARTMENT
  WHERE Dno=Dnumber AND MgrStartDate LIKE “198%”

Answer 2

hnkj

Question 3

R - S ( except ) is NOT EQUAL S - R

Answer 3

Rember this

Question 4

R1 divided by R2

Answer 4

R2 is a subset of R1
every attribute in r2 is in r1
and r2 is not equal to r1

The division results in tuples which appear in R in combination with
EVERY tuples in S

Identified usually by All and Every in the natural language sentence