MySQL提供了三大運算子來處理NULL值：

    ·IS NULL:當列的值是NULL,此運算子返回true;

    ·IS NOT NULL:當列的值不為NULL,此運算子返回true；

    ·<=>：比較操作符(不同於=運算子),當比較的兩個值為NULL時返回true;

關於NULL的條件比較運算子是比較特殊的。不能使用=NULL或!=NULL在列中查詢NULL值。

在MySQL中， NULL值與任何其他值的比較(即使是NULL)永遠返回false,即NULL=NULL返回false。

MySQL中處理NULL使用IS NULL 和 IS NOT NULL 運算子。

例項

[email protected]# mysql -u root -p password;
Enter password:*******
mysql> use RUNOOB;
Database changed
mysql> create table runoob_test_tbl
    -> (
    -> runoob_author varchar(40) NOT NULL,
    -> runoob_count  INT
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values ('RUNOOB', 20);
mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values ('菜鳥教程', NULL);
mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values ('Google', NULL);
mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values ('FK', 20);
 
mysql> SELECT * from runoob_test_tbl;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| RUNOOB        | 20           |
| 菜鳥教程  | NULL         |
| Google        | NULL         |
| FK            | 20           |
+---------------+--------------+
 

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

mysql> SELECT * FROM runoob_test_tbl WHERE runoob_count IS NULL;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鳥教程  | NULL         |
| Google        | NULL         |
+---------------+--------------+
2 rows in set (0.01 sec)
 
mysql> SELECT * from runoob_test_tbl WHERE runoob_count IS NOT NULL;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| RUNOOB        | 20           |
| FK            | 20           |
+---------------+--------------+
2 rows in set (0.01 sec)
 

使用PHP指令碼處理NULL值

<?php
$dbhost = 'localhost:3306';  // mysql伺服器主機地址
$dbuser = 'root';            // mysql使用者名稱
$dbpass = '123456';          // mysql使用者名稱密碼
$conn = mysqli_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
    die('連線失敗: ' . mysqli_error($conn));
}
// 設定編碼，防止中文亂碼
mysqli_query($conn , "set names utf8");
 
if( isset($runoob_count ))
{
   $sql = "SELECT runoob_author, runoob_count
           FROM  runoob_test_tbl
           WHERE runoob_count = $runoob_count";
}
else
{
   $sql = "SELECT runoob_author, runoob_count
           FROM  runoob_test_tbl
           WHERE runoob_count IS NULL";
}
mysqli_select_db( $conn, 'RUNOOB' );
$retval = mysqli_query( $conn, $sql );
if(! $retval )
{
    die('無法讀取資料: ' . mysqli_error($conn));
}

echo '<table border="1"><tr><td>作者</td><td>登陸次數</td></tr>';
while($row = mysqli_fetch_array($retval, MYSQL_ASSOC))
{
    echo "<tr>".
         "<td>{$row['runoob_author']} </td> ".
         "<td>{$row['runoob_count']} </td> ".
         "</tr>";
}
echo '</table>';
mysqli_close($conn);
?>