《MYSQL教程详解MySQL中的NULL值》要点：
本文介绍了MYSQL教程详解MySQL中的NULL值，希望对您有用。如果有疑问，可以联系我们。
MYSQL应用我们已经看到使用WHERE子句的SQL SELECT命令来从MySQL表获取数据.但是,当我们试图给的条件比较字段或列的值为NULL,它不能正常工作.
MYSQL应用为了处理这种情况,MySQL提供了三大运算符
MYSQL应用涉及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.
MYSQL应用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：
MYSQL应用假设一个表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:
MYSQL应用试试下面的例子：
MYSQL应用
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>
MYSQL应用可以看到=和!=不使用NULL值,如下所示：
MYSQL应用
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)
MYSQL应用要找到,其中tutorial_count列是或不是NULL的记录,查询应该这样写：
MYSQL应用
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)
MYSQL应用PHP脚本处理NULL值：
MYSQL应用可以使用IF ... ELSE条件准备的基础上操作NULL值的查询.
例子:
MYSQL应用下面的示例tutorial_count从外部,然后它与可在表中的值进行比较.
MYSQL应用
<?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);
?>
转载请注明本页网址：
http://www.vephp.com/jiaocheng/5233.html
