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Oracle/NULL value

NULL value

Soul-Learner 2008. 6. 14. 19:42
NOTE: Null In Oracle is an absence of information. A null can be assigned but it can not be equated with anything: Even itself.

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.

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