NULL Data in table
Null mean missing or unknown value. Null is not equal to zero or blank space data in a column. Null is also not same as empty string. So to match a Null value we can use IS NULL condition in our SQL statement to get the records from a database table.
For all our examples we have modified our student table by making class and mark column to accept null and changed the data to add some null values. You can download and install a copy of this table to test in your database. The SQL dump of student3 is available at the end of this page.
SELECT* FROM`student3` WHERE class IS NULL
This will display all the records which have NULL value for class column.
Similarly we can display all the records which do not have null value for the class. Like this
SELECT * FROM `student3` WHERE class IS NOT NULL
Now we will get all the records without having NULL value in class column
Making the column data to null
We can make all the class data ( or column ) equal to NULL by updating the record like this
update `student3` set class = null
We can't change the value of the column ( class here ) if the property is set to NOT NULL for the column. To make the column null for all the records we first have to make the table accept NULL value by updating the structure of the table.
ALTER TABLE `student3` CHANGE `class` `class` VARCHAR( 10 ) CHARACTER SET latin1 COLLATE latin1_swedish_ci NULL DEFAULT ''
After this change we can add NULL value to the record.
Deleting records with null value
We can delete records for which some column has null value. Here it is
DELETE FROM student3 WHERE class IS NULL
As we know null value means unknown data so how to use the value ? A situation may come where we have to multiply or apply any other mathematical calculation on all the data of a column ( say Mark in our student table ) which have some null data also. For this we have to use different sql commands like IFNULL, COALESCE, NVL. We will learn in next Part.
FNULL, COALESCE, NVL
Download sql dump of student3 table
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