@Jcook
oops, I did mistype that in my original post, it was _x0020_ that I tried for a space. 🙂 And here are my results testing empty filters and also column names with only 1 word...
So here is a screenshot of the action I'm using:

And the output (I've only include the first record here) when I leave the filters empty looks like this:
{"body":{"value":[{"@odata.etag":"","ItemInternalId":"6d269a8b-a6eb-4c26-80d5-c2d530517d31","ID":"163","Start time":"43866.7106481481","Completion time":"43866.7122685185","Name":"Bonnie","Outside CLIA Slide Total":"","Screening Date":"43831","Non-Gyn Smears, Cell Blocks":"","Non-Gyn Thinprep, Surepath, Cytospin":"","Gyn Thinprep FOV Only":"","Gyn Thinprep FOV + Manual":"","Gyn Thinprep Manual Only":"","Gyn Surepath FOV Only":"","Gyn Surepath FOV + Manual":"","Gyn Surepath Manual Only":"","QC Screened":"","Proficiency Testing, Educational Slides, QA Slides":"","Screening Hours":"0","Outside Screening Hours":"","Total non-screening hours":"8","Brief description of non-screening activities":"New Year's Day","Did you screen at other labs today?":"No","Total UCL CLIA Slides":"0","Total UCL+Outside Scr Hours":"0","UCL Sl/Hr":"**No screening recorded for UCL today.","Total UCL+Outside CLIA Slides":"0","UCL+Outside Sl/Hr":"**No screening recorded today.","Total UCL Hours Worked":"8","slide rate over":"","duplicate scr date":"","slide count over":"","scr hours over":"","Column1":"2020-02-05","sup alert":"", As an example, I have columns called Name, Column1, Screening Date. If I test entering one word column names separated by commas into the select query I get the correct output for those columns. But if I try to enter a column into it with 2 words like Screening Date I get a BadRequest error (see below.) So I've tried every way to format the name I can think to use it, the quotes and brackets etc. I just haven't stumbled upon the correct one for it yet.
So, when I select query for name,column1 I get this:
{
"value": [
{
"@odata.etag": "",
"ItemInternalId": "0adb76d7-cc84-4f8e-9e41-4b812a27fd85",
"Name": "Bonnie",
"Column1": "2020-02-05"
},
And if I select query for name,screening date (or even just screening date by itself), I get this:
{
"status": 400,
"message": "Term 'name,screening date' is not valid in a $select or $expand expression.\r\n inner exception: Term 'name,screening date' is not valid in a $select or $expand expression.\r\nclientRequestId: 575cefd1-4784-4be9-95ce-51459e2b45f1",
"error": {
"message": "Term 'name,screening date' is not valid in a $select or $expand expression.\r\n inner exception: Term 'name,screening date' is not valid in a $select or $expand expression."
},
"source": "excelonline-eus2.azconn-eus2.p.azurewebsites.net"
}