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>

可以看到=和!=不使用NULL值，如下所示：

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)

要找到，其中tutorial_count列是或不是NULL的記錄，查詢應(yīng)該這樣寫：

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)

PHP腳本處理NULL值：

可以使用IF ... ELSE條件準(zhǔn)備的基礎(chǔ)上操作NULL值的查詢。
例子:

下面的示例tutorial_count從外部，然后它與可在表中的值進(jìn)行比較。

<?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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