Forum Discussion
Good Source to Learn DAX
There are lots of sources
Start here with guided learning
https://powerbi.microsoft.com/en-us/learning/
MS have started a new learning site. Its new so there isn't much yet, but they are adding more.
https://docs.microsoft.com/en-us/learn/browse/?products=bizapps-power-bi
There is a free edx course
https://courses.edx.org/courses/course-v1:Microsoft+DAT207x+1T2017/course/
Curbal is a fantastic chanel ruthpozuelo has loads of videos on lots of Dax functions.
https://www.youtube.com/channel/UCJ7UhloHSA4wAqPzyi6TOkw
Powerbi tips have a good set of links to sites including learning
This resource is proving to be very well written!
https://bengribaudo.com/blog/2018/01/19/4321/power-query-m-primer-part4-variables-identifiers
I'd like to add the following great (free) DAX course from EnterpriseDNA (not affiliated)
http://portal.enterprisedna.co/p/ultimate-beginners-guide-to-dax
DAX fridays with Ruth from Curbal.com is also a great learning source.
https://www.youtube.com/channel/UCJ7UhloHSA4wAqPzyi6TOkw
DAX Patterns is a list of common patterns to solve common problems.
When it comes to books I would like to recommend "Supercharge Power BI: Power BI Is Better When You Learn to Write DAX" by Matt Allington. It contains a lot of exercises that will help you solidify your learning. It is a great introduction to DAX.
https://www.amazon.com/Supercharge-Power-BI-Better-Learn/dp/1615470522?tag=exccbs-20
The Definitive Guide to DAX is the DAX bible. Apparently a new edition is coming out in February 2019.
Mastering DAX Workshop from SQLBI is probably the most in depth training I have come across/taken. The modules are great as it helped me to not feel so overwhelemed by the amount of information. They have quizes at the end of each section, which I found helpful. Plus, I found them both to be entertaining and kept me interested.
Combine that with their book Definitive Guide to DAX, and if you are willing to put in the time you will be much better off!