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 |
|
|