Generally, we are using Fork n Join method ( divide the data into 2 links which goes to 2 Aggregator ) to calculate Sum and Count Rows of Data because Aggregator Stage is providing one type of aggregation at a time, You can Count the rows or can do some other aggregation.
Here, I have describe a way in which we can Count Rows and Calculate Sum in a Single Aggregator Stage with some limitations :-)
Design :
As per design, we used Column Generator and Aggregator Stage to calculate and count.
