我们已经看到SQL SELECT命令和WHERE子句一起使用,来从MySQL表中提取数据,但是,当我们试图给出一个条件,比较字段或列值设置为NULL,它确不能正常工作。
为了处理这种情况,MySQL提供了三大运算符
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IS NULL: 如果列的值为NULL,运算结果返回 true
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IS NOT NULL: 如果列的值不为NULL,运算结果返回 true
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<=>: 运算符比较值,(不同于=运算符)即使两个空值它返回 true
涉及NULL的条件是特殊的。不能使用= NULL或!= NULL来匹配查找列的NULL值。这样的比较总是失败,因为它是不可能告诉它们是否是true。 甚至 NULL = NULL 也是失败的。
要查找列的值是或不是NULL,使用IS NULL或IS NOT NULL。
在命令提示符,使用NULL值
假设在 test 数据库中的表 tcount_tbl 它包含两个列 tutorial_author 和 tutorial_count, 其中 tutorial_count 的值为NULL表明其值未知:
示例
试试下面的例子:
root@host# mysql -u root -p password; Enter password: mysql> use test; 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>
可以看到,= 及 != 不能与 NULL值不能正常工作(匹配)如下:
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)
要查找记录中,其中 tutorial_count 列的值是或不是NULL,查询应该这样写:
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)
在PHP脚本中处理NULL值
可以使用 if...else 条件来基于NULL值的查询。
示例
下面的示例,从外面使用 tutorial_count,然后在表中可用的值进行比较。
<?php $dbhost = 'localhost:3036'; $dbuser = 'root'; $dbpass = ''; $conn = mysql_connect($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('test'); $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); ?>