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We have seen the SQL SELECT command along with the WHERE clause to fetch data from a MySQL table, but when we try to give a condition, which compares the field or the column value to NULL, it does not work properly.
To handle such a situation, MySQL provides three operators −
-
IS NULL − This operator returns true, if the column value is NULL.
-
IS NOT NULL − This operator returns true, if the column value is not NULL.
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<=> − This operator compares values, which (unlike the = operator) is true even for two NULL values.
The conditions involving NULL are special. You cannot use = NULL or != NULL to look for NULL values in columns. Such comparisons always fail because it is impossible to tell whether they are true or not. Sometimes, 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 the Command Prompt
Assume that there is a table called tcount_tbl in the TUTORIALS database and it contains two columns namely tutorial_author and tutorial_count, where a NULL tutorial_count indicates that the value is unknown.
Example
Try the 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>
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)
To find the records where the tutorial_count column is or is not NULL, the queries should be written as shown in the following program.
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)
Handling NULL Values in a PHP Script
You can use the if…else condition to prepare a query based on the NULL value.
The following example takes the tutorial_count from outside and then compares it with the value available in the table.
Example
Copy and paste the following example as mysql_example.php −
<html>
<head>
<title>Handling NULL</title>
</head>
<body>
<?php
$dbhost = ''localhost'';
$dbuser = ''root'';
$dbpass = ''root@123'';
$dbname = ''TUTORIALS'';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $dbname);
$tutorial_count = null;
if($mysqli→connect_errno ) {
printf("Connect failed: %s<br />", $mysqli→connect_error);
exit();
}
printf(''Connected successfully.<br />'');
if( isset($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 NULL'';
}
$result = $mysqli→query($sql);
if ($result→num_rows > 0) {
while($row = $result→fetch_assoc()) {
printf("Author: %s, Count: %d <br />",
$row["tutorial_author"],
$row["tutorial_count"]);
}
} else {
printf(''No record found.<br />'');
}
$mysqli→close();
?>
</body>
</html>
Output
Access the mysql_example.php deployed on apache web server and verify the output.
Connected successfully. No record found.
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