Summarize grouping by year and month

Solution 1:

This is what pivot tables are for.

I have created an example using your data.

First, add a column to format the date as your monthly string, eg "2009-01" with the formula:

TEXT(A2, "YYYY-MM")

Then highlight the data, and choose "data" > "pivot table report..."

For Rows, select "month"

For Values, select "amount"

TADA! That's it!

For a quick overview of pivot tables, see this Google blog post.

Solution 2:

You can do it with an array formula, i put the formula in A2 and in A1, i put the month number:

=ARRAYFORMULA(SUMPRODUCT(1*(MONTH(Sheet1!$A$2:$A$6)=A1)*Sheet1!$B$2:$B$6))

See the doc in action in Google Spreadsheet

[EDIT] If you want both year and month:

=ARRAYFORMULA(SUMPRODUCT(1*(YEAR(Sheet1!$A$2:$A$8)=A2)*(MONTH(Sheet1!$A$2:$A$8)=B2)*Sheet1!$B$2:$B$8))