《MYSQL教程詳解MySQL中的NULL值》要點：
本文介紹了MYSQL教程詳解MySQL中的NULL值，希望對您有用。如果有疑問，可以聯(lián)系我們。
MYSQL應用我們已經看到使用WHERE子句的SQL SELECT命令來從MySQL表獲取數(shù)據.但是,當我們試圖給的條件比較字段或列的值為NULL,它不能正常工作.
MYSQL應用為了處理這種情況,MySQL提供了三大運算符
MYSQL應用涉及NULL條件是特殊的.不能使用 =NULL 或 !=NULL 尋找NULL值的列.這種比較總是告訴他們是否是真正的失敗,因為這是不可能的.即使是NULL=NULL失敗.
MYSQL應用如果要查找是或不是NULL的列,請使用IS NULL或IS NOT NULL.
在命令提示符下使用NULL值：
MYSQL應用假設一個表tcount_tbl,它包含了兩個的列stutorial_author和tutorial_count,其中一個tutorial_count為NULL 表示的值是未知的
例子:
MYSQL應用試試下面的例子：
MYSQL應用
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應用可以看到=和!=不使用NULL值,如下所示：
MYSQL應用
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應用要找到,其中tutorial_count列是或不是NULL的記錄,查詢應該這樣寫：
MYSQL應用
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應用PHP腳本處理NULL值：
MYSQL應用可以使用IF ... ELSE條件準備的基礎上操作NULL值的查詢.
例子:
MYSQL應用下面的示例tutorial_count從外部,然后它與可在表中的值進行比較.
MYSQL應用
<?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);
?>
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