Shopify import, I believe, does not allow to update data only by having the SKU.
If you are open to using apps, then our app Excelify has such a feature and would allow you to update products only by the SKU.
For example, you can upload a file with only two columns to update prices:
The same for most other product fields that can be updated such as tags, qty, adding images, and so on.
If you are interested, then for more info you can check out our tutorial about bulk updating products by SKU.
If you have any questions or issue, let me know here or contact our support directly by any means listed in our "Contact Us" page.
You can leave out the non-mandatory columns from the upload file.
Unfortunately the documentation on which field is mandatory and which isn't is not very accurate:
It's a trial and error thing you need to test yourself to find the proper method. I usually keep all the fields and use Excel to produce the desired upload file.
PM me if you need some advice.
Note, that the failsafes that work for uploading new products are off when updating existing products. A friend of mine managed to wipe the pictures of about 500 of his items because he made a mistake in the url column and Shopify did not check for the validity of the url-s for the update and wiped all the pictures. I strongly advise to always test your upload file with a few products first. There's the rollback option but I prefer to load things into a test store first before updating thousands of products.
Of course, you can always purchase some apps and gizmos to update stuff but I prefer the oldschool way.
My name is Alison, and I'm the Customer Success Specialist at Ablestar. I'm sorry for the late response on this, but I wanted to reach out and let you know about our 'Bulk Editor' app. Using it you can upload a spreadsheet where the first column is the identifying column (SKU will work), and then as many columns as you need to update. If you only need to update one field you would just need SKU and the field.
You can read more about spreadsheet edits here: https://support.ablestar.co/article/23-editing-products-by-spreadsheet. The app also has a free version available that allows for 5 free edits per month.
I you have any questions you can reach out to us directly at firstname.lastname@example.org.
Check out this post, it explains how to use the built-in CSV upload feature and edit only the columns you want to edit: