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：
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