云萌主云应用官方论坛

标题: 【转载】MySQL NULL 值处理（下） [打印本页]

作者: 简简单单 时间: 2017-2-22 11:07
标题: 【转载】MySQL NULL 值处理（下）
本帖最后由简简单单于 2017-2-22 11:09 编辑

以下实例中你可以看到 = 和 != 运算符是不起作用的：

mysql> SELECT * FROM tcount_tbl WHERE runoob_count = NULL;Empty set (0.00 sec)mysql> SELECT * FROM tcount_tbl WHERE runoob_count != NULL;Empty set (0.01 sec)

查找数据表中 runoob_count 列是否为 NULL，必须使用IS NULL和IS NOT NULL，如下实例：

mysql> SELECT * FROM tcount_tbl -> WHERE runoob_count IS NULL;+-----------------+----------------+| runoob_author | runoob_count |+-----------------+----------------+| mahnaz | NULL || Jen | NULL |+-----------------+----------------+2 rows in set (0.00 sec)mysql> SELECT * from tcount_tbl -> WHERE runoob_count IS NOT NULL;+-----------------+----------------+| runoob_author | runoob_count |+-----------------+----------------+| mahran | 20 || Gill | 20 |+-----------------+----------------+2 rows in set (0.00 sec)

使用PHP脚本处理 NULL 值

PHP脚本中你可以在 if...else 语句来处理变量是否为空，并生成相应的条件语句。

以下实例中PHP设置了$runoob_count变量，然后使用该变量与数据表中的 runoob_count 字段进行比较：

<?php$dbhost = 'localhost:3036';$dbuser = 'root';$dbpass = 'rootpassword';$conn = mysql_connect($dbhost, $dbuser, $dbpass);if(! $conn ){ die('Could not connect: ' . mysql_error());}if( isset($runoob_count )){ $sql = 'SELECT runoob_author, runoob_count FROM tcount_tbl WHERE runoob_count = $runoob_count';}else{ $sql = 'SELECT runoob_author, runoob_count FROM tcount_tbl WHERE runoob_count IS $runoob_count';}mysql_select_db('RUNOOB');$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['runoob_author']} <br> ". "Count: {$row['runoob_count']} <br> ". "--------------------------------<br>";} echo "Fetched data successfully\n";mysql_close($conn);?>

欢迎光临云萌主云应用官方论坛 (https://yunmengzhu.com/)