SQL每月平均总天数

Question

I have a SQL question. I am trying to find the average injection volume per month. Currently my code takes the sum of all days of injection, and divides them by the TOTAL DAYS in the month .

Sum(W1."INJECTION_VOLUME" /
EXTRACT(DAY FROM LAST_DAY(W1."INJECTION_DATE"))) AS "AVGINJ"

This is not what I wanted. I need to take the injection_volume and divide by the total days in the DATA .

ie. right now the data only 8 days of injection volume , lets say it is 3000.

So right now the sql is 3000/31.

I need to have it be 3000/8 (the total days in the data for the current month.)

Also, this should only be for the current month. All other completed months should be divided by the total days in the month.

Answer 1

In order to get the average of the data for the current month you will need to divide by the count in the month:

SUM(`W1`.`INJECTION_VOLUME` / COUNT(EXTRACT(YEAR_MONTH FROM `W1`.`INJECTION_DATE`)))

To get all other data as the full month you'll need to combine your code:

SUM(`W1`.`INJECTION_VOLUME` / EXTRACT(DAY FROM LAST_DAY(`W1`.`INJECTION_DATE`)))

With an IF . So something like this:

SUM(
    IF(
        EXTRACT(YEAR_MONTH FROM `W1`.`INJECTION_DATE`) = EXTRACT(YEAR_MONTH FROM NOW()), 
        `W1`.`INJECTION_VOLUME` / COUNT(EXTRACT(YEAR_MONTH FROM `W1`.`INJECTION_DATE`)), 
        `W1`.`INJECTION_VOLUME` / EXTRACT(DAY FROM LAST_DAY(`W1`.`INJECTION_DATE`)
    )
)

Note: this is untested and I'm not sure about the RDBMS you are using so you may need to change the code slightly to make it work.

Answer 2

Use

SELECT
SUM(W1.INJECTION_VOLUME) / COUNT(DISTINCT MyDateField)
FROM MyTable
WHERE X=Value

This gives you what you're after

SUM(W1.INJECTION_VOLUME) is the total volume for the dataset

Gives you the number of days, no matter how many records you have

COUNT(DISTINCT MyDateField)

So if you have 100 records but only 5 actual unique days in this time, this expression gives you 5

Note that this kind of calc is normally worked out with

SUM(A) / SUM(B)

not

SUM(A/B)

They give you completely different answers.