SQL> create table t1 (id number,col1 varchar2(16) default 'DEF' NOT NULL); Table created. SQL> insert into t1 (id,col1) values(1,NULL); insert into t1 (id,col1) values(1,NULL) * ERROR at line 1: ORA-01400: cannot insert NULL into ("JYC"."T1"."COL1") SQL> insert into t1 (id) values(1); 1 row created. SQL> commit; Commit complete. SQL> SELECT NVL2(NULLIF('CODA', 'SID'), 'SPANIEL', 'TERRIER') FROM DUAL; NVL2(NU ------- SPANIEL SQL> SELECT NULLIF('CODA', 'SID') FROM DUAL; NULL ---- CODA SQL> SELECT NULLIF('CODA', 'SID'), 'SPANIEL', 'TERRIER' FROM DUAL; NULL 'SPANIE 'TERRIE ---- ------- ------- CODA SPANIEL TERRIER SQL> SELECT NVL2(NULLIF('CODA', 'SID'), 'SPANIEL', 'TERRIER') FROM DUAL; NVL2(NU ------- SPANIEL SQL>