Handpng MySQL NULL Values

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.

<=> − This operator compares values, which (unpke 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)

Handpng 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>Handpng NULL</title>
   </head>
   <body>
      <?php
         $dbhost =  localhost ;
         $dbuser =  root ;
         $dbpass =  root@123 ;
         $dbname =  TUTORIALS ;
         $mysqp = new mysqp($dbhost, $dbuser, $dbpass, $dbname);
         $tutorial_count = null;
         if($mysqp->connect_errno ) {
            printf("Connect failed: %s<br />", $mysqp->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 = $mysqp->query($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 /> );
         }
         $mysqp->close();
      ?>
   </body>
</html>

Output

Access the mysql_example.php deployed on apache web server and verify the output.


Connected successfully.
No record found.

Handpng MySQL NULL Values

Using NULL values at the Command Prompt

Example

Handpng NULL Values in a PHP Script

Example

Output

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