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Hello
your formula in C6 cannot work due to a default address of the search field:
1°) the search field ;2008; and only the name of the sheet???
2°) and even if you use the field name nom2008 it will not work because B4 is not in the 1st column of the field
See this one:
=VLOOKUP($B4&"*";'2008'!B:C;2;1)

for your information
from this column B of 2008, you can retrieve in A the code and in C the amount
with in A4 this formula:

=IF(B4="";""INDEX(nom2008;MATCH(B4&"*";2008!B:B;0);1))

and in B4
=IF(B4="";""INDEX(nom2008;MATCH(B4&"*";2008!B:B;0);3))

That said, we assume that your 2008 sheet contains each reference line only once, as only the 1st found will be returned.
If however the aim was to find the cumulative by reference, use instead:

=IF(B4="";"";SUMIF(2008!B:B;B4&"*";2008!C:C)

kind regards
To err is human, to persist is diabolical.

I would like to warmly thank you all because my problem is solved thanks to you!

""Even if you use the field name nom2008, it won't work because B4 is not in the first column of the field ""

I swapped the columns and it worked!!

Thank you! Here is the link to the file
https://www.cjoint.com/?DEpxcu3ZCqC

As you can see, the formula is in C4, first sheet "by designations". =VLOOKUP($A4, nom2008, 3, 0)
The formula associates A4 (the nomenclature) with the consumption located in the named range nom2008, column 3 of the 2008 sheet.
What I would like: simply for =VLOOKUP($B4, nom2008, 3, 0) to work in order to associate the designations with the consumptions instead of the nomenclatures.

Not because the file looks like that, but I actually have 10,000 rows!!!

https://www.cjoint.com/?DEqfWl60Pct

I hope I followed your advice correctly. But unfortunately, it still doesn't work...