在MySQL中isnull()函數不能作為替代null值！
如下：
首先有個名字為business的表：
| SELECT ISNULL(business_name,'no business_name') AS bus_isnull FROM business WHERE id=2 |
直接運行就會報錯：
錯誤代碼： 1582
| Incorrect parameter count in the call to native function 'isnull' |
所以，isnull()函數在mysql中就行不通了。可以用ifnull()和coalesce()代替。如下：
使用ifnull()函數：
| SELECT IFNULL(business_name,'no business_name') AS bus_ifnull FROM business WHERE id=2 |
運行結果：
當查詢的值不為null時：
| SELECT IFNULL(business_name,'no business_name') AS bus_ifnull FROM business WHERE id=1 |
結果如下：
使用coalesce()函數：
| SELECT COALESCE(business_name,'no business_name') AS bus_coalesce FROM business WHERE id=2 |
結果如下：
當查詢值不為null時：
| SELECT COALESCE(business_name,'no business_name') AS bus_coalesce FROM business WHERE id=1 |
其中：coalesce()還可以返回第一個不為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) |
結果如下：
同樣，is null 和is not null 也是用在where后面。
| SELECT * FROM business WHERE business_name IS NULL |
結果如下：
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