https://www.shanelynn.ie/summarising-aggregation-and-grouping-data-in-python-pandas/

Comment¶

1. The order of elements within each group are preserved (as the original order).

2. groupby works exactly the same on index if the index is named.

3. The order of columns in groupby matters if you want unstack the results later.

4. groupby works on columns too and it can group by some level of a MultiIndex.

Comments¶

Even though Spark DataFrame/SQL APIs do not distinguish cases of column names, the columns saved into HDFS are case-sensitive!