The PostgreSQL NULL is the term used to represent a missing value. A NULL value in a table is a value in a field that appears to be blank.
A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different from a zero value or a field that contains spaces.
Syntax
The basic syntax of using NULL while creating a table is as follows −
CREATE TABLE COMPANY( ID INT PRIMARY KEY NOT NULL, NAME TEXT NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(50), SALARY REAL );
Here, NOT NULL signifies that column should always accept an explicit value of the given data type. There are two columns where we did not use NOT NULL. Hence, this means these columns could be NULL.
A field with a NULL value is one that has been left blank during record creation.
Example
The NULL value can cause problems when selecting data, because when comparing an unknown value to any other value, the result is always unknown and not included in the final results. Consider the following table, COMPANY having the following records −
ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 1 Paul 32 California 20000.0 2 Allen 25 Texas 15000.0 3 Teddy 23 Norway 20000.0 4 Mark 25 Rich-Mond 65000.0 5 David 27 Texas 85000.0 6 Kim 22 South-Hall 45000.0 7 James 24 Houston 10000.0
Let us use the UPDATE statement to set few nullable values as NULL as follows −
testdb=# UPDATE COMPANY SET ADDRESS = NULL, SALARY = NULL where ID IN(6,7);
Now, COMPANY table should have the following records −
id | name | age | address | salary ----+-------+-----+-------------+-------- 1 | Paul | 32 | California | 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | | 7 | James | 24 | | (7 rows)
Next, let us see the usage of IS NOT NULL operator to list down all the records where SALARY is not NULL −
testdb=# SELECT ID, NAME, AGE, ADDRESS, SALARY FROM COMPANY WHERE SALARY IS NOT NULL;
The above given PostgreSQL statement will produce the following result −
id | name | age | address | salary ----+-------+-----+------------+-------- 1 | Paul | 32 | California | 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 (5 rows)
The following is the usage of IS NULL operator which will list down all the records where SALARY is NULL −
testdb=# SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM COMPANY
WHERE SALARY IS NULL;
The above given PostgreSQL statement will produce the following result −
id | name | age | address | salary ----+-------+-----+---------+-------- 6 | Kim | 22 | | 7 | James | 24 | | (2 rows)