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 of column value is NULL.
IS NOT NULL: operator returns true of column value is not NULL.
<=> operator compare 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 tcount_tbl in TUTORIALS database and it contains two columns tutorial_author and tutorial_count, where a NULL tutorial_count indicates that the value is unknown:
Example:
Try out following examples:
root@host# mysql -u root -p password;
Enter password:*******
mysql> use TUTORIALS;
Database changed
mysql> create table tcount_tbl
-> (
-> tutorial_author varchar(40) NOT NULL,
-> tutorial_count INT
-> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO tcount_tbl
-> (tutorial_author, tutorial_count) values ('mahran', 20);
mysql> INSERT INTO tcount_tbl
-> (tutorial_author, tutorial_count) values ('mahnaz', NULL);
mysql> INSERT INTO tcount_tbl
-> (tutorial_author, tutorial_count) values ('Jen', NULL);
mysql> INSERT INTO tcount_tbl
-> (tutorial_author, tutorial_count) values ('Gill', 20);
mysql> SELECT * from tcount_tbl;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran | 20 |
| mahnaz | NULL |
| Jen | NULL |
| Gill | 20 |
+-----------------+----------------+
4 rows in set (0.00 sec)
mysql>
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You can see that = and != do not work with NULL values as follows:
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count = NULL;
Empty set (0.00 sec)
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count != NULL;
Empty set (0.01 sec)
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To find records where the tutorial_count column is or is not NULL, the queries should be written like this:
mysql> SELECT * FROM tcount_tbl
-> WHERE tutorial_count IS NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahnaz | NULL |
| Jen | NULL |
+-----------------+----------------+
2 rows in set (0.00 sec)
mysql> SELECT * from tcount_tbl
-> WHERE tutorial_count IS NOT NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran | 20 |
| Gill | 20 |
+-----------------+----------------+
2 rows in set (0.00 sec)
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Handling NULL Values in PHP Script:
You can use if...else condition to prepare a query based on NULL value.
Example:
Following example take tutorial_count from outside and then compare 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 tutorial_author, tutorial_count
FROM tcount_tbl
WHERE tutorial_count = $tutorial_count';
}
else
{
$sql = 'SELECT tutorial_author, tutorial_count
FROM tcount_tbl
WHERE tutorial_count IS $tutorial_count';
}
mysql_select_db('TUTORIALS');
$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['tutorial_author']} <br> ".
"Count: {$row['tutorial_count']} <br> ".
"--------------------------------<br>";
}
echo "Fetched data successfully\n";
mysql_close($conn);
?>
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