Re: CHECK col A not NULL if col B='x'

T E Schmitz <mailreg@numerixtechnology.de> writes:
> I noticed a table constraint can be added via ALTER TABLE. Is it correct 
> that a column constraint cannot be added via the ALTER TABLE other than 
> by dropping and adding the column? (I am using psql 7.4.2.)

There is no difference between table and column constraints in PG.  Just
write it as a table constraint in ALTER TABLE.
        regards, tom lane