mysql中IFNULL()和COALESCE()函数替代null

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

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直接运行就会报错：

错误代码： 1582
Incorrect parameter count in the call to native function 'isnull'

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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

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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

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结果如下：

使用coalesce()函数：

SELECT COALESCE(business_name,'no business_name') AS bus_coalesce FROM business WHERE id=2

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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

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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

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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)

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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

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结果如下：

SELECT * FROM business WHERE business_name IS NOT NULL

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