How can i make a column be foreign key that refers to multiple tables?

Question

i want the column which is a foreign key to refer to a table conditionally, Im using psql database,

sample 
       id ( primary key not null ) 
       A (id references table A.id nullable)
       B (id references table B.id nullable)     // both cannot be null and only A or B //can have value
       externalRef (enum of varchar stores A | B)

currently in my sample table i have 2 columns A and B but tomorrow if i want to add C, D, E so on i have to create those columns too, how can i solve this and make it better ?

I have a vague idea about the solution but not sure if its implementable ?, so i wanna keep 2 columns one will be type and other will the the refid (refid will be the foreign key column that will refer to table that type has) How can i add this condition ??