Thank you.
I've found out that the following is working (function node into node-red-node-mysql node):
client = 'client1'
project = 'project1'
meter_id = 'meter_id1'
date = msg.payload;
value = 1
msg.topic = "INSERT INTO `digitalempire`.`meterdata` (`client`, `project`, `meter_id`, `date`, `value`) VALUES ('client1, 'project1', 'meter1', '2020-08-09 09:03:55', '10');"
return msg;
Now I only need to figure out how to implement the placeholders.