![](https://csdnimg.cn/release/download_crawler_static/12104845/bg5.jpg)
5
1
2
3
4
5
6
1
2
3
(
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1
2
3
1 2
4.2
E-R
1 2
3
E-R
E-R 1 2 3
1 2
A
B A
1
2
3
4.3
1 2 (3) 4
4.4
1 2