Why did MySQL SUM IF quit working in 2021 - mysql

Question

This code quit working in 2021 after working in 2020:
SELECT name, SUM(CASE WHEN YEARWEEK(prod_date) = YEARWEEK(now()- INTERVAL 1 WEEK) THEN gas_prod ELSE NULL END) AS LastWeek FROM daily_prod GROUP BY name
The code now calculates a larger number than occurred last week. Any reason why a new year would cause this code to work differently?

Answer 1

First, using the sum( case/when ) is a bad choice here. Your query is going against your ENTIRE daily_prod table, EVERY RECORD, but only summing when within the given yeardate() period based on whatever the current date is. Instead of doing that, build out a WHERE clause to just get the records you care about. The YearWeek() function is based on a Sunday to Saturday week schedule. So, for example this week is from Jan 31 at 12:00am (midnight) up to, but not including Feb 7th at 12:00am midnight. This includes everything up to Feb 6th at 11:59:59pm.
So, by doing embedded MySQL variables you can build as a "from" table alias as I will show. First, lets get whatever the current NOW() is which is inclusive of hour/minute/second and strip down to just the date portion. Ex: 2021-02-04 # 01:42am truncates down to just 2021-02-04 12:00am
select cast( now() as date )
From that, now, we need to go to the first of the week, represented as Sunday. For this, we use the day of the week and subtract 1 so it is a zero-based value where Sunday = 0, Saturday = 6. So Thursday, normally returns 5, subtract 1 = 4. February 4 - 4 days = Jan 31 which is the first of the current week.
select date_sub( [result of sql above], interval dayofweek( [result of sql above] ) -1 day )
Now, since you want the entire week of data, in this scenario, your data would be from Jan 31 at 12:00am in the morning up to Feb 6th (Saturday) at 11:59:59PM. So, take the above beginning of the week and add 1 week to it bringing you to Feb 7th.
select date_add( [result of 2nd query], interval 1 week )
Yes, this is intentional because now in your final query you can query LESS THAN Feb 7th.
So, by using a where on your data source, you only get those records, AND, if your table has an index on the transaction date, can be optimized for the query. To use inline variables, we can just use the results of these as part of the query such as:
SELECT
dp.name,
SUM( dp.gas_prod ) CurrwntWeekGas
FROM
daily_prod dp,
( select
-- first, variable #nd = Now as a date only
#nd := cast( now() as date ),
-- from the #nd, subtract 0-based day of week
-- example: Thursday is 5th day of week -1 = 4 days to subtract
#fow := date_sub( #nd, interval dayofweek( #nd ) -1 day ),
-- finally from the first of the week, add 1 week to the END point for the query
#nextWeek := date_add( #fow, interval 1 week )
) sqlvars
where
-- only grab records greater or equal to the first day of the week
dp.prod_date >= #fow
-- and ALSO LESS then the the beginning of NEXT week
AND dp.prod_date < #nextWeek
GROUP BY
dp.name
So, as the from clause alias "sqlvars" is processed, it creates the 3 "#" variables to define the beginning of week and beginning of NEXT week values. Those can then be applied to the where clause and limit just the records you need, not the full table.
If you really want the results of the prior week from the week you are in... Ex: the total from Jan 24th to Jan 30th since the current week has not completed, then just change your dayofweek() -1 to dayofweek() -8 to get the entire prior week, not the CURRENT week you are in the middle of now.