Set Theoretic Operation (Cont.)

Assignment and Alias
Let R be a table and let Head(R)=A1…An. Assume that B1…Bn are n attributes such that Domain(Bi)=Domain(Ai) for all i, 1≤i≤n. We define a new table S, whose heading is Head(S)=B1…Bn, by writing the assignment 
S( B1, ..., Bn ) := R( R1, ..., An )
The content of the new table S is exactly the same as the content of the old table R. S is an alias of the table or R is all attributes are identical between the two tables: S:=R.

An Example
R
A B C
a1 b1 c1
a1 b2 c3
a2 b1 c2
S
A B C
a1 b1 c1
a1 b1 c2
a1 b2 c3
a3 b2 c3
T := (R∪S)–(R∩S)
⇒
T1 := (R∪S)
T2 := (R∩S)
T := T1–T2
T
A B C
a1 b1 c2
a2 b1 c2
a3 b2 c3




      “Where exactly are you taking me, doctor?” the patient asked.    
      “To the morgue,” the doctor replied.    
      “What?” The patient panicked. “But I’m not dead yet!”    
      “And we’re not there yet,” the doctor said.