They are also different from a design perspective:
e.g. CREATE TABLE t ( id INTEGER NOT NULL, name CHARACTER(40), CONSTRAINT t_PK PRIMARY KEY (id) );
CREATE UNIQUE INDEX t_AK1 ON t (name);
Looks like:
\d t Table "public.t" Column | Type | Modifiers --------+---------------+----------- id | integer | not null name | character(40) | Indexes: "t_pk" PRIMARY KEY, btree (id) "t_ak1" UNIQUE, btree (name)
Lets insert some data:
op=# insert into t(id, name ) values ( 1, 'Hello'); INSERT 0 1
op=# insert into t( id, name) values ( 2, ''); INSERT 0 1
op=# insert into t( id, name) values ( 3, '');
ERROR: duplicate key value violates unique constraint "t_ak1"
Now lets try with null:
op=# insert into t( id, name) values (4, null );
INSERT 0 1
op=# insert into t( id, name) values (5, null);
INSERT 0 1
This is allowed.
Soooooo: nulls are not trivial strings nor the reverse.
Cheers