Q2是该查询改写的表名。

Q2 is the rewritten table name by the query rewrite.

youdao

请注意，在查询10中多获得了一行，这是因为SQL的group by函数还为null值生成一个组，我们的示例表中有一名员工没有办公室信息。

Note that we get an extra row from Query 10, because SQL's group by also produces a group for NULL values, and there is one employee in our sample table with no office information.

youdao