本文介绍了MYSQL教程详解MySQL中的NULL值,希望对您有用。如果有疑问,可以联系我们。
MYSQL应用我们已经看到使用WHERE子句的SQL SELECT命令来从MySQL表获取数据.但是,当我们试图给的条件比较字段或列的值为NULL,它不能正常工作.
MYSQL应用为了处理这种情况,MySQL提供了三大运算符
IS NULL: 此运算符返回true,当列的值是NULL.
IS NOT NULL: 运算符返回true,当列的值不是NULL.
<=> 操作符比较值(不同于=运算符)为ture,即使两个NULL值
MYSQL应用涉及NULL条件是特殊的.不能使用 =NULL 或 !=NULL 寻找NULL值的列.这种比较总是告诉他们是否是真正的失败,因为这是不可能的.即使是NULL=NULL失败.
MYSQL应用如果要查找是或不是NULL的列,请使用IS NULL或IS NOT NULL.
在命令提示符下使用NULL值:
MYSQL应用假设一个表tcount_tbl,它包含了两个的列stutorial_author和tutorial_count,其中一个tutorial_count为NULL 表示的值是未知的
例子:
MYSQL应用试试下面的例子:
MYSQL应用
- root@host# mysql -u root -p password;
- Enter password:*******
- mysql> use TUTORIALS;
- 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',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>
- MYSQL应用可以看到=和!=不使用NULL值,如下所示:
- MYSQL应用
- 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)
- MYSQL应用要找到,其中tutorial_count列是或不是NULL的记录,查询应该这样写:
- MYSQL应用
- 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)
MYSQL应用PHP脚本处理NULL值:
MYSQL应用可以使用IF ... ELSE条件准备的基础上操作NULL值的查询.
例子:
MYSQL应用下面的示例tutorial_count从外部,然后它与可在表中的值进行比较.
MYSQL应用
- <?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: {$row['tutorial_count']} <br> ".
- "--------------------------------<br>";
- }
- echo "Fetched data successfully ";
- mysql_close($conn);
- ?>