Data not showing properly

Hi all I am quite new to Grafana and would be very grateful if someone can answer my question:

I am currently using data from Influxdb and needs to create a query as below:
SELECT (pipe_1+pipe_2+pipe_3) FROM water_flow_data WHERE $timeFilter

The purpose of this query is to show the total amount of water flow from the pipes as a time-series data.
The problem is that when pipe_3 does not have water flow it will not have data, and the above query will not function properly. Is is possible to create a query like the above one but when pipe_3 does not have any data it can still show the value of pipe_1+pipe_2?

Much appreciated!