test: improve function test coverage (#1) #6

Job	Run time
Build documentation	28s
Lint (black, isort, flake8, mypy, import-linter)	43s
Tests (Python 3.13)	21s
Tests (Python 3.11)	22s
Functional tests (MSSQL + MySQL + Postgres, no Oracle)	1m 1s
Tests (Python 3.9)	27s
Tests (Python 3.12)	25s
Tests (Python 3.14)	24s
Tests (Python 3.10)	22s
Functional tests (Oracle, on-demand)	0s
	4m 33s

Provide feedback