Add column from matching value in another row - Query Editor

[horseyride]

This should be very simple, but I'm stumped.
I want to add a column by matching the current value of [Last Year-Week] and pulling in the associated [Sales] value

Year-Week	Last-Year-Week	Sales
2017-01	2016-01	100
2017-02	2016-02	200
2017-03	2016-03	300
2017-04	2016-04	400
2018-01	2017-01	500
2018-02	2017-02	600
2018-03	2017-03	700
2018-04	2017-04	800

<tbody>
</tbody>

to:

Year-Week	Last-Year-Week	Sales	Previous Sales
2017-01	2016-01	100	null
2017-02	2016-02	200	null
2017-03	2016-03	300	null
2017-04	2016-04	400	null
2018-01	2017-01	500	100
2018-02	2017-02	600	200
2018-03	2017-03	700	300
2018-04	2017-04	800	400

<tbody>
</tbody>

= Table.AddColumn(Source, "Previous Sales", each if [#"Year-Week"] = [Last Year-Week] then [Sales] else 0 )
No idea why this doesn't work

 

[gazpage]

Duplicate your query and then merge them together using the Year-Week from one query and Last-Year-Week from the other.