Help needed with data preparation (using M)

[Cassis]

Hey ppl!

I've received a new task and need to learn M fast.

Problem is, I have a "simple" problem to solve but didn't learn enough M yet to solve it in a timely manner.

Hope you guys can help.

I have a table called "Update" with a single column "When" and single row containing one date.

Like this:

TABLE Update
When
28/02/2020

This date is dynamic, obtained by a query.

I have a second table "Water_Consumption" with a date column "Reading" and a column named "m3" formatted as number (integer). Like this:

TABLE Water_Consumption
Reading                m3
22/12/2019          210
22/01/2020          208
22/02/2020             0
22/03/2020             0
22/04/2020             0

My problem:
I need to change 0 to null when the date is greater than the date in "When".

So in the current example, my Water_Consumption table would look like:

TABLE Water_Consumption
Reading          m3
22/12/2019     210
22/01/2020     208
22/02/2020        0
22/03/2020     null
22/04/2020     null

This is as far as I could go, it works but requires me a manual update every month:

Table.ReplaceValue(#"Previous_step", each [m3], each if [Reading] <= #date(2020, 2,28) then [m3] else null, Replacer.ReplaceValue, {"m3"} )

Problem is, I need to make it update automatically so my current "solution" doesn't cut it.

Can you guys please advise?

Thanks in advance!

[sturlaws · Accepted solution]

Hi @Cassis,

it seems like you are taking on the advanced editor in Power Query. Perhaps an easier way to do this is to first create a new column with the value from Update, and then create another column according to the logic you have. Once that is completed you can remove the original m3-column and the When-column.

If you still want to do this in Advanced editor, I think this will work for you:

= Table.ReplaceValue(#"Previous step", each [m3], each if [Reading] <= List.First(Update[When]) then [m3] else null, Replacer.ReplaceValue, {"m3"} )

Cheers,
Sturla

If this post helps, then please consider Accepting it as the solution. Kudos are nice too.

[Cassis]

Thanks mate, your solution solved the issue!

Also thanks for your suggestion, I believe it would be an easier way to solve the problem but I was afraid the impact in performance would hurt the model as a whole - that's why I opted to use the editor and dedicate an effort to learn M.