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MySQL NULL 值如何处理？

时间：2021-07-01 10:21:17 帮助过：8人阅读

# mysql -u root -p password;Enter password:*******mysql> use OPENKETANG; Database changedmysql> create table openketang_test_tbl -> ( -> openketang_author varchar(40) NOT NULL, -> openketang_count INT -> ); Query OK, 0 rows affected (0.05 sec)mysql> INSERT INTO openketang_test_tbl (openketang_author, openketang_count) values (‘OPENKETANG‘, 20); mysql> INSERT INTO openketang_test_tbl (openketang_author, openketang_count) values (‘阿里云大学‘, NULL); mysql> INSERT INTO openketang_test_tbl (openketang_author, openketang_count) values (‘Google‘, NULL); mysql> INSERT INTO openketang_test_tbl (openketang_author, openketang_count) values (‘FK‘, 20); mysql> SELECT * from openketang_test_tbl; +---------------+--------------+ | openketang_author | openketang_count | +---------------+--------------+ | OPENKETANG | 20 | | 阿里云大学 | NULL | | Google | NULL | | FK | 20 | +---------------+--------------+ 4 rows in set (0.01 sec)

以下实例中你可以看到 = 和 != 运算符是不起作用的：

mysql> SELECT * FROM openketang_test_tbl WHERE openketang_count = NULL;

Empty set (0.00 sec)mysql> SELECT * FROM openketang_test_tbl WHERE openketang_count != NULL;

Empty set (0.01 sec)

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

mysql> SELECT * FROM openketang_test_tbl WHERE openketang_count IS NULL;

 +---------------+--------------+

 | openketang_author | openketang_count |

  +---------------+--------------+

  | 阿里云大学  | NULL         |

  | Google        | NULL         |

  +---------------+--------------+

  2 rows in set (0.01 sec) mysql> SELECT * from openketang_test_tbl WHERE

  openketang_count IS NOT NULL;

  +---------------+--------------+

  | openketang_author | openketang_count |

  +---------------+--------------+

  | OPENKETANG        | 20           |

  | FK            | 20           |

   +---------------+--------------+

   2 rows in set (0.01 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 $aliyun_count 变量，然后使用该变量与数据表中的 aliyun_count 字段进行比较：

MySQL ORDER BY 测试：

<?php$dbhost = ‘localhost:3306‘; 

// mysql服务器主机地址$dbuser = ‘root‘;           

// mysql用户名$dbpass = ‘123456‘;         

// mysql用户名密码$conn = mysqli_connect($dbhost, $dbuser, $dbpass);

if(! $conn ){ die(‘连接失败: ‘ . mysqli_error($conn));}

// 设置编码，防止中文乱码

mysqli_query($conn , "set names utf8");

if( isset($openketang_count )){ $sql = "SELECT openketang_author, openketang_count

FROM openketang_test_tbl WHERE openketang_count = $openketang_count";}

else{ $sql = "SELECT openketang_author, openketang_count FROM openketang_test_tbl          

WHERE openketang_count IS NULL";}

mysqli_select_db( $conn, ‘OPENKETANG‘ );

$retval = mysqli_query( $conn, $sql );

if(! $retval ){ die(‘无法读取数据: ‘ . mysqli_error($conn));}

echo ‘<h2>阿里云大学 IS NULL 测试<h2>‘;

echo ‘<table border="1"><tr><td>作者</td><td>登陆次数</td></tr>‘;

while($row = mysqli_fetch_array($retval, MYSQL_ASSOC)){    echo "<tr>".

"<td>{$row[‘openketang_author‘]} </td> ".<td>{$row[‘openketang_count‘]} </td> ".

"</tr>";}echo ‘</table>‘;mysqli_close($conn);?>