How do I impose a CONSTRAINT on a TEXT column in PostgreSQL 12? I want a column with a single text character as a classifier so it's not worth the overhead creating another whole table and relating to it.

How do I constrain the column to a list of acceptable characters?

E.g. how would I constrain column my_classifier CHAR(1) NOT NULL, to one of [abcd]?
Here is one method to do it:

create table test
(
  dd char NOT NULL,
  constraint dd_c check ( dd in ('a', 'b', 'c', 'd'))
);