Content

1. Tips

Tips

• May not drop table with referential constraints
  • Drop something in source OK; problem dropping in referenced table
  • Add something in referenced table OK; problem dropping in source
• Foreign keys must reference primary key or other superkeys of the referenced table

• UNION and OR are not interchangeable; UNION will discard duplicates

  • OR

      SELECT E.name
      FROM Employees E, Works_in W
      WHERE E.employee_id= W.employee_id
      AND(W.department_id = 1 OR W.department_id = 3)
    

  • OR

      SELECT E.name
      FROM Employees E, Works_in W
      WHERE E.employee_id = W.employee_id 
      AND W.department_id = 1
    
      UNION
    
      SELECT E.name
      FROM Employees E, Works_in W
      WHERE E.employee_id = W.employee_id 
      AND W.department_id = 3
    

• INTERSECT = JOIN, EXCEPT = NOT IN
• UNKNOWN is like 0.5 in comparisons
• Assertions are checked every time the table is updated