Precedence of Relational Operations

( L × M ) where L.city = M.city
∦
L × M where L.city = M.city
∥
L × ( M where L.city = M.city )
Precedence Operators Symbol
Highest PROJECT R[ ]
SELECT R where C
Times ×
JOIN
DIVIDEBY		 ∞
÷
INTERSECTION ∩
Lowest UNION
DIFFERENCE		 ∪
–

Query: Find those cities where we have either customers who have a discount of less than 10% or agents who make a commission of less than 6%.

customer
cid cname city discnt
c001 TipTop Duluth 10.00
c002 Basics Dallas 12.00
c003 Applied Dallas 12.00
c004 ACME Duluth 12.00
c006 ACME Kyoto 0.00
agent
aid aname city percent
a01 Smith New York 6
a02 Jones Newark 6
a03 Brown Tokyo 7
a04 Gray New York 6
a05 Otasi Duluth 5
a06 Smith Dallas 5 


(CUSTOMER where discnt < 10)[city] ∪
(AGENT where percent < 6)[city]
 		⇒
city
Dallas
Duluth
Kyoto




      Being deeply loved by someone gives you strength,    
      while loving someone deeply gives you courage.    
      — Lao Tzu