You have the syntax correct, but are confused about how to use ALIASes in SQL code to clarify which specific variable you are referencing.
The ALIAS is a ONE word string that refers to a table mentioned in your SQL statement. By default the alias will be the member name of the dataset you are referencing. If you want to use a different alias then add the alias after the table (dataset) name in your statement. This is critical when you are using either the same dataset twice or two datasets with the same member name from two different libraries.
In this example I have used LEFT and RIGHT as the aliases.
proc sql;
create table chop_dx_joined as
select *
from chop.diagnosis LEFT
full join chop.visit_diagnosis RIGHT
on LEFT.dx_key = RIGHT.dx_key
;
run;
Note this is standard SQL syntax. So it will work just as well in PosgreSQL's implementation of SQL.