在MySQL中isnull()函数不能作为替代null值!
如下： mysql中替代null的IFNULL()与COALESCE()函数详解
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 mysql中替代null的IFNULL()与COALESCE()函数详解
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