CODE
Oracle Code Library
JOBS
Find Or Post Oracle Jobs
FORUM
Oracle Discussion & Chat
PSOUG Home Code Snippets Oracle Reference Oracle Functions PSOUG Forum Oracle Blogs Bookmark and Share
 
 Search the Reference Library pages:  

Free
Oracle Magazine
Subscriptions
and Oracle White Papers

Oracle NULLs
Version 11.1
 
NOTE: Null In Oracle is an absence of information. A null can be assigned but it cannot be equated with anything, including itself. NULL values represent missing or unknown data. NULL values are not an integer, a character, or any other specific data type. Note that NULL is not the same as an empty data string or the numerical value '0'.

While this behavior is ANSI compliant it is not similar to the behavior in many other commercial RDBMS products.
A simple SELECT statement to use for demonstrating the properties of NULL SELECT COUNT(*)
FROM all_tables
WHERE 1 = 1;
A NULL is not equal to a NULL SELECT COUNT(*)
FROM all_tables
WHERE NULL = NULL;
A NULL cannot be not equal to a NULL SELECT COUNT(*)
FROM all_tables
WHERE NULL <> NULL;
A NULL is does not equal an empty string SELECT COUNT(*)
FROM all_tables
WHERE NULL = '';

A NULL can  be used in an INSERT
CREATE TABLE test (
test1   NUMBER(10),
test2   VARCHAR2(20));

INSERT INTO test
(test1, test2)
VALUES
(1, NULL);

INSERT INTO test
(test1, test2)
VALUES
(NULL, 'A');

SELECT *
FROM test;
 
NOTE: Null is a state of being that can be interrogated as to whether it does or does not exist.
A simple SELECT based on a column with a NULL SELECT *
FROM test
WHERE test1 IS NULL;

SELECT *
FROM test
WHERE test1 IS NOT NULL
A NULL can be used in an UPDATE UPDATE test
SET test1 = '2'
WHERE test2 IS NULL;

SELECT *
FROM test;

UPDATE test
SET test2 = 'B'
WHERE test2 IS NOT NULL;

SELECT *
FROM test;
A column can be updated to not contain a value UPDATE test
SET test1 = NULL
WHERE ROWNUM = 1;

SELECT *
FROM test;
NULL can be used as part of the WHERE clause criteria in a DELETE Statement DELETE FROM test
WHERE test1 IS NULL;

SELECT *
FROM test;

Understand the implications of NULL
CREATE TABLE t (
col1 NUMBER(3),
col2 NUMBER(3),
col3 NUMBER(3));

desc t

INSERT INTO t
(col1, col2, col3)
VALUES
(1, NULL, NULL);

INSERT INTO t
(col1, col2, col3)
VALUES
(NULL, 2, NULL);

INSERT INTO t
(col1, col2, col3)
VALUES
(NULL, NULL, 3);

INSERT INTO t
(col1, col2, col3)
VALUES
(4, 4, 4);

COMMIT;

SELECT *
FROM t;

SELECT SUM(RESULT_TMP) RESULT
FROM (
  SELECT col1 - (col2 + col3) RESULT_TMP 
  FROM t);

SELECT SUM(col1) - (SUM(col2) + SUM(col3)) RESULT
FROM t;

Note: For any row that has one of the values null, the entire row sums to null and is not included in the second query but the other columns in the row contribute to the sums in the query. So the first query includes more terms than the second.
 
Related Topics
Miscellaneous Functions: NVL
Miscellaneous Functions: NVL2
Select
 
Home      :      Code Library      :      Sponsors      :      Privacy      :      Terms of Use      :      Contact Us [69 users online]    © 2010 psoug.org