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Mysql IFNULL操作
项目中用到的,当SQL查询某个字段为空的时候,查询结果中设置其值为默认值。最笨的方法当然是对查询结果进行处理了,遍历查询结果,当为空的时候,设置其值:
代码如下 复制代码
$len=count($result);
for($i=0;$i $var = $result[$i]['name'];
if(!$var){
$result[$i]['name']='default_name';
}
}
如上办法,不仅费时,还??隆H绻?茉?ql语句中直接解决多好啊。
SQL中有ISNULL方法,介绍如下:
ISNULL
使用指定的替换值替换 NULL。
语法
ISNULL ( check_expression , replacement_value )
参数
check_expression
将被检查是否为 NULL的表达式。check_expression 可以是任何类型的。
replacement_value
在 check_expression 为 NULL时将返回的表达式。replacement_value 必须与 check_expresssion 具有相同的类型。
但在Mysql中,isnull只是用来判断是否为空,不能实现替换功能,照上面写的话,会直接报错(Incorrect parameter count in the call to native function 'isnull' Errornumber:1582 )。
那么Mysql中如何实现SQL中的ISNULL方法呢?
IFNULL( check_expression , replacement_value ),实现了SQL中的ISNULL方法。
见下sql语句
代码如下 复制代码
mysql> SELECT IFNULL(1,0);
+-------------+
| IFNULL(1,0) |
+-------------+
| 1 |
+-------------+
1 row in set
由于 expr1 是 1,不为 NULL,所以函数返回 1. 我们可以再试下,如果让 expr1 为 NULL,是否能返回第二个参数呢? 不妨让 expr1 = 1/0, 由于除数为0,结果就为 NULL 了。
代码如下 复制代码
mysql> SELECT IFNULL(1/0,'NowaMagic');
+-------------------------+
| IFNULL(1/0,'NowaMagic') |
+-------------------------+
| www.111cn.net |
+-------------------------+
1 row in set
前面的参数为 NULL,就返回第二个参数 www.111cn.net 了。
这个函数不难理解,为什么要介绍这个函数呢?这是为了后面讲述 MySQL 下的高级黑客技术做的基础知识准备,你可以先试着理解这么一句 hack SQL
代码如下 复制代码
UPDATE table SET views = '1' WHERE id = -2441 OR (ORD(MID((SELECT IFNULL(CAST(FirstName AS CHAR),0x20) FROM nowamagic.`tb2` ORDER BY id LIMIT 1,1),2,1))>112)#
Mysql的IN操作
见如下SQL语句
代码如下 复制代码
select product
from ProductParameter
where val = 19116 OR val = 19127 OR val = 19128 OR val = 19119
OR了很多是不是觉得很繁琐呀?用IN就解决了:
代码如下 复制代码
select product
from ProductParameter
where val in (19116, 19127, 19128, 19119))
更多详细内容请查看:http://www.111cn.net/database/mysql/57520.htm
bitsCN.com
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