详解MySQL中的NULL值_MySQL

为了处理这种情况，MySQL提供了三大运算符

1. IS NULL: 此运算符返回true，当列的值是NULL。
2. IS NOT NULL: 运算符返回true，当列的值不是NULL。
3. <=> 操作符比较值（不同于=运算符）为ture，即使两个NULL值

涉及NULL条件是特殊的。不能使用 =NULL 或 !=NULL 寻找NULL值的列。这种比较总是告诉他们是否是真正的失败，因为这是不可能的。即使是NULL=NULL失败。

如果要查找是或不是NULL的列，请使用IS NULL或IS NOT NULL。
在命令提示符下使用NULL值：

假设一个表tcount_tbl，它包含了两个的列stutorial_author和tutorial_count，其中一个tutorial_count为NULL 表示的值是未知的
例子:

试试下面的例子：

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的记录，查询应该这样写：

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脚本处理NULL值：

可以使用IF ... ELSE条件准备的基础上操作NULL值的查询。
例子:

下面的示例tutorial_count从外部，然后它与可在表中的值进行比较。

<?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']} 
 ".
     "Count: {$row['tutorial_count']} 
 ".
     "--------------------------------
";
} 
echo "Fetched data successfully\n";
mysql_close($conn);
?>