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. |
