How to Allow null in on column If i Create Composite Unique

Question

Hi All,
I have table EMP_APPROVAL having COL1,COL2,COL3
COL1,COL2 are Mandatory and COL3 Most of the times null.
I CREATED COMPOSITE UNIQUE KEY BUT IT ALLOWS ONLY ON RECORD NULL VALUE IN COL3.
is there any suggestion!
Thanks,
Nr

Answer 1

If COL3 is NULL, the index has no entry for that row. If COL3 is not null, you could do this:SQL> CREATE TABLE T(
  COL1 NUMBER NOT NULL,
  COL2 NUMBER NOT NULL,
  COL3 NUMBER
table T created.
SQL> CREATE UNIQUE INDEX IDX_T ON T(NVL2(COL3,COL1,NULL),NVL2(COL3,COL2,NULL),COL3)
unique index IDX_T created.
SQL> INSERT INTO T
SELECT LEVEL, LEVEL, DECODE(MOD(LEVEL,2),0,LEVEL, NULL)
FROM DUAL
CONNECT BY LEVEL <= 1000
1,000 rows inserted.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(OWNNAME => USER, TABNAME => 'T')
anonymous block completed
SQL> SELECT /*+ gather_plan_statistics */ * FROM T
WHERE (NVL2(COL3,COL1,NULL),NVL2(COL3,COL2,NULL),COL3) = ((8, 8, 8))
COL1 COL2 COL3
   8    8    8
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'))
| Id  | Operation                   | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers |                                                                                                                                                                                                             
|   0 | SELECT STATEMENT            |       |      1 |        |      1 |00:00:00.01 |       3 |                                                                                                                                                                                                             
|   1 |  TABLE ACCESS BY INDEX ROWID| T     |      1 |      1 |      1 |00:00:00.01 |       3 |                                                                                                                                                                                                             
|*  2 |   INDEX UNIQUE SCAN         | IDX_T |      1 |      1 |      1 |00:00:00.01 |       2 |                                                                                                                                                                                                             
Predicate Information (identified by operation id):                                                                                                                                                                                                                                                         
2 - access("T"."SYS_NC00004$"=8 AND "T"."SYS_NC00005$"=8 AND "COL3"=8)To me, this would make little sense. I would create a normal index on COL1,COL2,COL3 for use in queries (whether COL3 is NULL or not). I would use the above function-based index just for selective uniqueness.