我們已經看到使用WHERE子句的SQL SELECT命令來從MySQL表獲取數據。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。
為了處理這種情況，MySQL提供了三大運算符
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。
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：
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:
試試下面的例子：
root@host# mysql -u root -p password;Enter password:*******mysql> use TUTORIALS;Database changedmysql> 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條件準備的基礎上操作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);?>新聞熱點
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