MySQL NULL Values Handling

We have seen SQL SELECT command along with WHERE clause to fetch data from MySQL table, but when we try to give a condition, which compare field or column value to NULL, it does not work properly.
To handle such situation MySQL provides three operators

• IS NULL: operator returns true if column value is NULL.
• IS NOT NULL: operator returns true if column value is not NULL.
• <=>: operator compares values, which (unlike the = operator) is true even for two NULL values.

Conditions involving NULL are special. You cannot use = NULL or != NULL to look for NULL values in columns. Such comparisons always fail because it's impossible to tell whether or not they are true. Even NULL = NULL fails.
To look for columns that are or are not NULL, use IS NULL or IS NOT NULL.

Using NULL values at Command Prompt:

Suppose a table javatcount_tbl in JAVATUTORIALS database and it contains two columns javatutorial_author and javatutorial_count, where a NULL tutorial_count indicates that the value is unknown:

Example:

Try out the following examples:

root@host# mysql -u root -p password;
Enter password:*******
mysql> use JAVATUTORIALS;
Database changed
mysql> create table javatcount_tbl
    -> (
    -> javatutorial_author varchar(40) NOT NULL,
    -> javatutorial_count  INT
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO javatcount_tbl
    -> (javatutorial_author, javatutorial_count) values ('gemini', 20);
mysql> INSERT INTO javatcount_tbl
    -> (javatutorial_author, javatutorial_count) values ('ghandi', NULL);
mysql> INSERT INTO javatcount_tbl
    -> (javatutorial_author, javatutorial_count) values ('karhik', NULL);
mysql> INSERT INTO javatcount_tbl
    -> (javatutorial_author, javatutorial_count) values ('Gill', 20);

mysql> SELECT * from javatcount_tbl;
+---------------------+--------------------+
| javatutorial_author | javatutorial_count |
+---------------------+--------------------+
| gemini              |                 20 |
| ghandi              |               NULL |
| karthik             |               NULL |
| Gill                |                 20 |
+---------------------+--------------------+
4 rows in set (0.00 sec)

mysql>

You can see that = and != do not work with NULL values as follows:

mysql> SELECT * FROM javatcount_tbl WHERE javatutorial_count = NULL;
Empty set (0.00 sec)
mysql> SELECT * FROM javatcount_tbl WHERE javatutorial_count != NULL;
Empty set (0.01 sec)

To find records where the tutorial_count column is or is not NULL, the queries should be written like this:

mysql> SELECT * FROM javatcount_tbl 
    -> WHERE javatutorial_count IS NULL;
+---------------------+--------------------+
| javatutorial_author | javatutorial_count |
+---------------------+--------------------+
| ghandi              |               NULL |
| Karthik             |               NULL |
+---------------------+--------------------+
2 rows in set (0.00 sec)
mysql> SELECT * from tcount_tbl 
    -> WHERE tutorial_count IS NOT NULL;
+---------------------+--------------------+
| javatutorial_author | javatutorial_count |
+---------------------+--------------------+
| gemini              |                 20 |
| Gill                |                 20 |
+---------------------+--------------------+
2 rows in set (0.00 sec)

Handling NULL Values in PHP Script:

You can use if...else condition to prepare a query based on NULL value.

Example:

Following example takes tutorial_count from outside and then compares it with the value available in the table.

<?php
$dbhost = 'localhost:3036';
$dbuser = 'root';
$dbpass = 'rootpassword';
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
  die('Could not connect: ' . mysql_error());
}
if( isset($tutorial_count ))
{
   $sql = 'SELECT javatutorial_author, javatutorial_count
           FROM  javatcount_tbl
           WHERE javatutorial_count = $tutorial_count';
}
else
{
   $sql = 'SELECT javatutorial_author, javatutorial_count
           FROM  javatcount_tbl
           WHERE javatutorial_count IS $tutorial_count';
}

mysql_select_db('JAVATUTORIALS');
$retval = mysql_query( $sql, $conn );
if(! $retval )
{
  die('Could not get data: ' . mysql_error());
}
while($row = mysql_fetch_array($retval, MYSQL_ASSOC))
{
    echo "Author:{$row['javatutorial_author']}  <br> ".
         "Count: {$row['javatutorial_count']} <br> ".
         "--------------------------------<br>";
} 
echo "Fetched data successfully\n";
mysql_close($conn);
?>

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