我们已经看到SQL SELECT 命令以及 WHERE 子句从MySQL表中获取数据,但是当我们尝试给出一个比较该字段的条件时或者列值为 NULL ,它无法正常工作.
为了处理这种情况,MySQL提供了三个运算符 :
IS NULL : 如果列值为NULL,则此运算符返回true.
IS NOT NULL : 如果列值不为NULL,则此运算符返回true.
<=> : 此运算符比较值(与=运算符不同)即使对于两个NULL值也是如此.
涉及NULL的条件是特殊的.您不能使用= NULL 或!= NULL 来查找列中的NULL值.这种比较总是失败,因为无法判断它们是否真实.有时,偶数NULL = NULL失败.
要查找是或否为NULL的列,请使用 IS NULL 或 IS NOT NULL .
假设TUTORIALS数据库中有一个名为 tcount_tbl 的表它包含两列,即 tutorial_author 和 tutorial_count ,其中NULL tutorial_count表示该值未知.
尝试以下示例 :
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>
您可以看到=和!= 不能使用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)
要查找tutorial_count列为或非NULL的记录,应按如下所示编写查询以下程序.
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)
你可以使用 if ... else 根据NULL值准备查询的条件.
以下示例采用 tutorial_count 从外部,然后将其与表中可用的值进行比较.
<?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']} <br> ". "Count: {$row['tutorial_count']} <br> ". "--------------------------------<br>"; } echo "Fetched data successfully\n"; mysql_close($conn); ?>