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SQLite - NULL Values
SQLite 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 than a zero value or a field that contains spaces.
Syntax
Following is the basic syntax of using NULL while creating a table.
SQLite> 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 the column should always accept an explicit value of the given data type. There are two columns where we did not use NOT NULL which 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 with 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 UPDATE statement to set a few nullable values as NULL as follows −
sqlite> UPDATE COMPANY SET ADDRESS = NULL, SALARY = NULL where ID IN(6,7);
Now, COMPANY table will have 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 7 James 24
Next, let us see the usage of IS NOT NULL operator to list down all the records where SALARY is not NULL.
sqlite> SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM COMPANY
WHERE SALARY IS NOT NULL;
The above SQLite statement will produce the following result −
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
Following is the usage of IS NULL operator, which will list down all the records where SALARY is NULL.
sqlite> SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM COMPANY
WHERE SALARY IS NULL;
The above SQLite statement will produce the following result.
ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 6 Kim 22 7 James 24