单项选择题
如下图所示，两个关系R1和R2，它们经过哪种运算后得到R3
R1
|
A |
B |
C |
|
a |
1 |
x |
|
c |
2 |
y |
|
B |
D |
E |
|
1 |
m |
j |
|
2 |
n |
k |
|
A |
B |
C |
D |
E |
|
a |
1 |
x |
m |
j |
|
c |
2 |
y |
n |
k |
单项选择题
|
A |
B |
C |
|
a |
1 |
x |
|
c |
2 |
y |
|
B |
D |
E |
|
1 |
m |
j |
|
2 |
n |
k |
|
A |
B |
C |
D |
E |
|
a |
1 |
x |
m |
j |
|
c |
2 |
y |
n |
k |