1. Example Schema: Employee (ENO, ENAME, TITLE)
Works_On (ENO, PNO, RESP, DUR)
Project (PNO, PNAME)
Query:
“Find the names of employees other than J. John
who worked on the CAD/CAM project for either
one or two years”.
The SQL expression:
SELECT ENAME
FROM Project, Employee, Works_On
WHERE Works_On.ENO = Employee.ENO
AND Works_On.PNO = Project.PNO
AND Employee.ENAME  ‘J. John’
AND Project.PNAME = ‘CAD/CAM’
AND (Project.DUR = 12 OR Project.DUR = 24)
Project
Select
PNO
Join
ENO
Project
Works-On
Employee
Figure 1: Operator Tree
PNO
ENO
Project
Works-On
Employee
Employee
Project
Works-On
Figure 2: Another Operator Tree
Figure 3: Operator Tree after applying the restrictions