County Subdivision By Census Tract

I'd say Geocorr is the go-to application for obtaining correspondences for any pairing of census geographies. You can definitely use it to get this info.

Note: in case you were not aware, there is not an exact nesting relationship between county subdivisions and tracts. Many county subdivisions include parts of multiple tracts and vice versa. But Geocorr will still give you a complete listing of all of these relationships along with info about the population in each area of intersection.

I haven't yet updated Geocorr with the new 2020 geographies, FYI. Probably later this year at the soonest.

You could do this with the geoheader files from the PL-94 (redistricting) release:

- Select all the blocks (sumlev 750) 

- keep the STATE, COUNTY,TRACT, and COUSUB columns

- reduce to unique rows

- combine STATE, COUNTY,TRACT, for unique tract ID and STATE, COUSUB for unique county subdivision ID

If you use python, I've got this code to simplify reading in the data

https://gist.github.com/JoeGermuska/8da7a3e5e86600f26a48f2d3ce68910b

or you could do it with csvgrep and csvcut from the csvkit tools

https://csvkit.readthedocs.io/en/latest/

Right, I'd forgotten you may be looking for the 2020 relationships, and Geocorr doesn't have those yet. (Looking forward to that, Glenn!)

Joe Germuska shared another good idea, but you wouldn't need to go down to the block level to get this relationship. Summary Level 730 reports parts of census tracts by county subdivision, voting district, and place. You could download 2020 data for this level through IPUMS NHGIS...

In the NHGIS data finder, first select a year filter for 2020.

Then, in the Geographic Levels filter pop-up window, click on "SHOW COMPOUND GEOGRAPHIC LEVELS" in the upper right, and then click on "SMALL AREA STATISTICAL UNITS" on left. Then select the tract level that's "by State--County--County Subdivision--Place/Remainder"...

After that, you'll see a list of 2020 Redistricting Data tables. You could select any of those, proceed through the rest of the interface, and download the data. The resulting data file will contain data for tract parts with codes indicating which county subdivision each part is in.