在mysql中isnull()函数不能作为替代null值！
如下：
首先有个名字为business的表： 
SELECT ISNULL(business_name,'no business_name') AS bus_isnull FROM business WHERE id=2直接运行就会报错：
错误代码： 1582
Incorrect parameter count in the call to native function 'isnull'所以，isnull()函数在mysql中就行不通了。可以用ifnull()和coalesce()代替。如下：
使用ifnull()函数：
SELECT IFNULL(business_name,'no business_name') AS bus_ifnull FROM business WHERE id=2运行结果： 
当查询的值不为null时：
SELECT IFNULL(business_name,'no business_name') AS bus_ifnull FROM business WHERE id=1结果如下： 
使用coalesce()函数：
SELECT COALESCE(business_name,'no business_name') AS bus_coalesce FROM business WHERE id=2结果如下： 
当查询值不为null时：
SELECT COALESCE(business_name,'no business_name') AS bus_coalesce FROM business WHERE id=1
其中：coalesce()还可以返回第一个不为null的值。如下：
SELECT COALESCE(business_name,district_id,id) AS bus_coalesce FROM business WHERE id=2
那么，isnull()在mysql中怎么用呢？答案就是用在where后面。如下：
SELECT * FROM business WHERE ISNULL(business_name)结果如下： 
同样，is null 和is not null 也是用在where后面。
SELECT * FROM business WHERE business_name IS NULL结果如下：
SELECT * FROM business WHERE business_name IS NOT NULL