mysqli

Khóa học miễn phí MySQLi – Handling NULL Values nhận dự án làm có lương

Posted on by user

MySQLi – Handling 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 (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.

Khóa học lập trình tại Toidayhoc vừa học vừa làm dự án vừa nhận lương: Khóa học lập trình nhận lương tại trung tâm Toidayhoc

user