CREATE TABLE testtab (
  label VARCHAR(30) PRIMARY KEY,
  a VARCHAR(10),
  b VARCHAR(10)
);
INSERT INTO testtab VALUES('both null',NULL,NULL);
INSERT INTO testtab VALUES('a null',NULL,'b not null');
INSERT INTO testtab VALUES('b empty string','a','');
INSERT INTO testtab VALUES('a null, b empty string',NULL,'');
INSERT INTO testtab VALUES('equal non-empty strings','x','x');
INSERT INTO testtab VALUES('non-equal non-empty strings','x','y');
SELECT * FROM testtab;
SELECT * FROM testtab WHERE a=b;
SELECT * FROM testtab WHERE a<>b;
SELECT * FROM testtab WHERE b IS NULL;
SELECT * FROM testtab WHERE b IS NOT NULL;
Table dropped.
                                                                                                               
SQL>   2    3    4    5
Table created.
                                                                                                               
SQL>
1 row created.
                                                                                                               
SQL>
1 row created.
                                                                                                               
SQL>
1 row created.
                                                                                                               
SQL>
1 row created.
                                                                                                               
SQL>
1 row created.
 
SQL>
1 row created.
 
SQL>
LABEL                          A          B
------------------------------ ---------- ----------
both null
a null                                    b not null
b empty string                 a
a null, b empty string
equal non-empty strings        x          x
non-equal non-empty strings    x          y
 
6 rows selected.
 
SQL>
LABEL                          A          B
------------------------------ ---------- ----------
equal non-empty strings        x          x
 
SQL>
LABEL                          A          B
------------------------------ ---------- ----------
non-equal non-empty strings    x          y
 
SQL>
LABEL                          A          B
------------------------------ ---------- ----------
both null
b empty string                 a
a null, b empty string
 
SQL>
 
LABEL                          A          B
------------------------------ ---------- ----------
a null                                    b not null
equal non-empty strings        x          x
non-equal non-empty strings    x          y