How to calculate percent change between two columns in SQL?

Question

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Currently learning SQL (using BigQuery) and I'm trying to calculate the percentage change between values from two different columns (ridership_2013 and ridership_2018 - both columns are integers). Based on the screenshot above, here is what I attempted to do:

select station_name, ridership_2013, ridership_2018,(ridership_2013-ridership_2018)/ridership_2013*100 AS 5yr_change
from bigquery-public-data.new_york_subway.subway_ridership_2013_present;

I'm getting the result of:

division by zero: -3558 / 0

Where am I going wrong and how do I properly calculate percentage change? Thank you.

Answer 1

Just use the NULLIF function to replace the denominator to NULL when ridership_2013 is zero.

SELECT
  station_name,
  ridership_2013,
  ridership_2018,
  (ridership_2013 - ridership_2018) / NULLIF(ridership_2013, 0) * 100 AS 5yr_change
FROM
  bigquery-public-data.new_york_subway.subway_ridership_2013_present;