Not all indexes on the left-hand side of the equal sign need to have a match, but at most can have one. If there is nothing for the left DataFrame index to align to, the resulting value will be missing. Let's create an example where this happens. We will use only the first three rows of the max_dept_sal Series to create a new column:

>>> employee['MAX_SALARY2'] = max_dept_sal['BASE_SALARY'].head(3)
>>> employee.MAX_SALARY2.value_counts()
140416.0    29
100000.0    11
64251.0      5
Name: MAX_SALARY2, dtype: int64


>>> employee.MAX_SALARY2.isnull().mean()
.9775

The operation completed successfully but filled in salaries for only three of the departments. All the other departments that did not appear in the first three rows of the max_dept_sal Series resulted in a missing value.