《MYSQL教程MySQL NULL 值處理實例詳解》要點：
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MySQL NULL 值處理MYSQL必讀
我們已經(jīng)知道MySQL使用 SQL SELECT 命令及 WHERE 子句來讀取數(shù)據(jù)表中的數(shù)據(jù),但是當(dāng)提供的查詢條件字段為 NULL 時,該命令可能就無法正常工作.MYSQL必讀
為了處理這種情況,MySQL提供了三大運算符:MYSQL必讀
關(guān)于 NULL 的條件比較運算是比較特殊的.你不能使用 = NULL 或 != NULL 在列中查找 NULL 值 .MYSQL必讀
在MySQL中,NULL值與任何其它值的比較(即使是NULL)永遠返回false,即 NULL = NULL 返回false .MYSQL必讀
MySQL中處理NULL使用IS NULL和IS NOT NULL運算符.MYSQL必讀
在命令提示符中使用 NULL 值MYSQL必讀
以下實例中假設(shè)數(shù)據(jù)庫 TUTORIALS 中的表 tcount_tbl 含有兩列 tutorial_author 和 tutorial_count, tutorial_count 中設(shè)置插入NULL值.MYSQL必讀
實例MYSQL必讀
嘗試以下實例:MYSQL必讀
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>
以下實例中你可以看到 = 和 != 運算符是不起作用的：MYSQL必讀
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)
查找數(shù)據(jù)表中 tutorial_count 列是否為 NULL,必須使用IS NULL和IS NOT NULL,如下實例：MYSQL必讀
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)
使用PHP腳本處理 NULL 值MYSQL必讀
PHP腳本中你可以在 if...else 語句來處理變量是否為空,并生成相應(yīng)的條件語句.MYSQL必讀
以下實例中PHP設(shè)置了$tutorial_count變量,然后使用該變量與數(shù)據(jù)表中的 tutorial_count 字段進行比較：MYSQL必讀
<?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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