我们已经看到SQL SELECT 命令以及 WHERE 子句从MySQLi表中获取数据,但是当我们尝试给出条件时,将字段或列值与 NULL ,它无法正常工作.
为了处理这种情况,MySQLi提供了三个运算符
IS NULL : 如果列值为NULL,则运算符返回true.
IS NOT NULL : 如果列值不为NULL,则运算符返回true.
< => : 运算符比较值,即(对于=运算符),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属实.偶数NULL = NULL失败.
要查找NULL或非NULL的列,请使用IS NULL或IS NOT NULL.
假设TUTORIALS数据库中有一个表 tutorials_bks ,它包含两列 id 和 name ,其中NULL名称表示该值未知去;
尝试以下示例 :
root@host# mysql -u root -p password; Enter password:******* mysql> use TUTORIALS; Database changed mysql> INSERT INTO tutorials_bks (book) values(''); Query OK, 1 row affected (0.08 sec) mysql> INSERT INTO tutorials_bks (book) values(''); Query OK, 1 row affected (0.08 sec) mysql> INSERT INTO tutorials_bks (book) values(''); Query OK, 1 row affected (0.08 sec) mysql> INSERT INTO tutorials_bks (book) values('NULL'); Query OK, 1 row affected (0.08 sec) mysql> INSERT INTO tutorials_bks (book) values('NULL'); Query OK, 1 row affected (0.08 sec) mysql> INSERT INTO tutorials_bks (book) values('NULL'); Query OK, 1 row affected (0.08 sec) +----+---------+ | id | book | +----+---------+ | 1 | java | | 2 | java | | 3 | html | | 4 | c++ | | 5 | Android | | 6 | | | 7 | | | 8 | | | 9 | NULL | | 10 | NULL | | 11 | NULL | +----+---------+ 11 rows in set (0.00 sec) mysql>
要查找name列为或不为NULL的记录,查询应该写成这样 :
mysql> SELECT * FROM tutorials_bks WHERE book = "null"; +----+------+ | id | book | +----+------+ | 9 | NULL | | 10 | NULL | | 11 | NULL | +----+------+ 3 rows in set (0.00 sec) mysql> SELECT * FROM tutorials_bks WHERE book ! = "null"; +----+---------+ | id | book | +----+---------+ | 1 | java | | 2 | java | | 3 | html | | 4 | c++ | | 5 | Android | | 6 | | | 7 | | | 8 | | +----+---------+ 8 rows in set (0.00 sec) mysql> SELECT * FROM tutorials_bks WHERE book = ""; +----+------+ | id | book | +----+------+ | 6 | | | 7 | | | 8 | | +----+------+ 3 rows in set (0.01 sec)
您可以使用 if ... else 条件来准备基于NULL值的查询.
以下示例从外部获取tutorial_count,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); ?>