Project
 Output relation contains all rows, but only the specified columns.
 Duplicates are eliminated, because relations are sets.
R
X 
Y 
Z 
1  eee  10 
1  eee  20 
2  eee  30 
2  fff  40 
3  fff  50 
4  fff  60 
4  fff  70 
project(R, [X, Y])
X 
Y 
1  eee 
2  eee 
2  fff 
3  fff 
4  fff 
 In the projected rows, there are duplicates:
 (1, eee) occurs twice.
 (4, fff) occurs twice.
 There is one occurrence of each in the result.
