Write a query to display the meter_number from the meter table which got maximum total_units during 8PM for a day in the electricity_reading.

Sure, here is a SQL query that should give you the desired result:

SELECT meter_number 
FROM meter 
JOIN electricity_reading 
ON meter.id = electricity_reading.meter_id 
WHERE TIME(electricity_reading.reading_time) = '20:00:00' 
GROUP BY meter_number 
ORDER BY SUM(electricity_reading.total_units) DESC 
LIMIT 1;

Here's a step-by-step explanation of the query:

1. SELECT meter_number FROM meter: This part of the query selects the meter_number column from the meter table.

2. JOIN electricity_reading ON meter.id = electricity_reading.meter_id: This part of the query joins the meter table with the electricity_reading table based on the condition that the id in the meter table matches the meter_id in the electricity_reading table.

3. WHERE TIME(electricity_reading.reading_time) = '20:00:00': This part of the query filters the records to only include those where the reading_time is 8PM.

4. GROUP BY meter_number: This part of the query groups the results by meter_number.

5. ORDER BY SUM(electricity_reading.total_units) DESC: This part of the query orders the results in descending order based on the sum of total_units for each meter_number.

6. LIMIT 1: This part of the query limits the results to the top 1 record, which will be the meter_number with the maximum total_units at 8PM.