简介：我们已经知道MySQL使用 SQL SELECT 命令及 WHERE 子句来读取数据表中的数据,但是当提供的查询条件字段为 NULL 时，该命令可能就无法正常工作。为了处理这种情况，MySQL提供了三大运算符:IS NULL: 当列的值是NULL,此 ...
我们已经知道MySQL使用 SQL SELECT 命令及 WHERE 子句来读取数据表中的数据,但是当提供的查询条件字段为 NULL 时，该命令可能就无法正常工作。为了处理这种情况，MySQL提供了三大运算符:
在命令提示符中使用 NULL 值以下实例中假设数据库 RUNOOB 中的表 tcount_tbl 含有两列 runoob_author 和 runoob_count, runoob_count 中设置插入NULL值。实例尝试以下实例: root@host# mysql -u root -p password;Enter password:*******mysql> use RUNOOB;Database changedmysql> create table tcount_tbl -> ( -> runoob_author varchar(40) NOT NULL, -> runoob_count INT -> );Query OK, 0 rows affected (0.05 sec)mysql> INSERT INTO tcount_tbl -> (runoob_author, runoob_count) values ("mahran", 20);mysql> INSERT INTO tcount_tbl -> (runoob_author, runoob_count) values ("mahnaz", NULL);mysql> INSERT INTO tcount_tbl -> (runoob_author, runoob_count) values ("Jen", NULL);mysql> INSERT INTO tcount_tbl -> (runoob_author, runoob_count) values ("Gill", 20);mysql> SELECT * from tcount_tbl;+-----------------+----------------+| runoob_author | runoob_count |+-----------------+----------------+| mahran | 20 || mahnaz | NULL || Jen | NULL || Gill | 20 |+-----------------+----------------+4 rows in set (0.00 sec)mysql>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: {$row["runoob_count"]} <br> ". "--------------------------------<br>";} echo "Fetched data successfully";mysql_close($conn);?>本文仅代表作者个人观点，不代表巅云官方发声，对观点有疑义请先联系作者本人进行修改，若内容非法请联系平台管理员，邮箱2522407257@qq.com。更多相关资讯，请到巅云www.yx10011.com学习互联网营销技术请到巅云建站www.yx10011.com。 |
