A quick ways to determine the major and minor version numbers of Apex.
And since we're only dealing with integers here, no problems with NLS settings (e.g. invalid number errors when converting '4.2' on a Dutch Apex installation, which expects '4,2').

As a side note: you can get the Apex patch version (if it exists) with a query like this: select max(patch_version) as patch_version from apex_patches where patch_number = (select max(patch_number) from apex_patches)

		4.2.1	...	4.2.4	4.2.5	4.2.6		5.0	...	5.0.3	5.0.4		5.1	5.1.1	5.1.2
12.2.0.1.0		NOK	NOK	NOK	NOK	NOK		NOK	NOK	NOK	OK		OK	OK	OK
12.1.0.2.0		NOK	NOK	NOK	OK	OK		OK	OK	OK	OK		OK	OK	OK
12.1.0.1.0		NOK?	NOK?	NOK?	OK	OK		OK	OK	OK	OK		OK	OK	OK
11.2.0.4.0		OK	OK	OK	OK	OK		OK	OK	OK	OK		OK	OK	OK
11.2.0.3.0		OK	OK	OK	OK	OK		OK	OK	OK	OK		NOK?	NOK?	NOK?
11.2.0.2.0		OK	OK	OK	OK	OK		OK	OK	OK	OK		NOK?	NOK?	NOK?
11.2.0.1.0		OK	OK	OK	OK	OK		OK	OK	OK	OK		NOK?	NOK?	NOK?
11.1.0.7.0		OK	OK	OK	OK	OK		OK	OK	OK	OK		NOK?	NOK?	NOK?
11.1.0.6.0		OK	OK	OK	OK	OK		OK	OK	OK	NOK		NOK	NOK	NOK
10.2.0.5.0		OK	OK	OK	OK	OK		OK	OK	OK	NOK		NOK	NOK	NOK
10.2.0.4.0		OK	OK	OK	OK	OK		OK	OK	OK	NOK		NOK	NOK	NOK

While playing around with pluggable databases in 12c a few years ago, I wondered what different versions of Apex could be deployed at the same time on different database versions. The idea being that having multiple versions of Apex running at the same time (and accessing the same data) would be a great help during application testing and upgrading.
Getting this info was not as simple as expected, so I thought I'd share my findings with you. So the list is "best effort" and it is not complete - I skipped Apex and database versions that are already antiques.

In the table above, "NOK?" indicates a combination of versions that might or might not be supported; the documentation is not always clear or consistent. To be safe, regard this as a "NOK".
Also note that a "NOK" doesn't mean the combination won't work; it might but it is not supported. For example, on two development servers I've had Apex 5.0.2 running on database 12.2.0.1.0 without problems.

Sources used:
https://mikedietrichde.com/2017/05/02/is-your-apex-version-certified-with-your-database-release/
https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=355341795729425&id=1344948.1&_afrWindowMode=0&_adf.ctrl-state=d04kwub2_4

Region

Identification
	Sequence	20
	Title	Apex version (regular expressions)
	Type	Report
Source
	Source Type	SQL_QUERY
	Region Source	select version_no , to_number( regexp_substr(version_no, '[^.]+', 1, 1) ) as v1 , to_number( regexp_substr(version_no, '[^.]+', 1, 2) ) as v2 , to_number( regexp_substr(version_no, '[^.]+', 1, 3) ) as v3 , to_number( regexp_substr(version_no, '[^.]+', 1, 4) ) as v4 , to_number( regexp_substr(version_no, '[^.]+', 1, 5) ) as v5 from apex_release