D3 documentation说：

The enter selection merges into the update selection when you append
or insert. This approach reduces code duplication between enter and
update. Rather than applying operators to both the enter and update
selection separately,you can now apply them to the update selection
after entering the nodes. In the rare case that you want to run
operators only on the updating nodes,you can run them on the update
selection before entering new nodes.

我不明白第一句中的短语合并的含义.有人可以解释这一点吗？ (也许我错过了与某些标准数据库术语的连接？)