Sql Update使用WHEN和WHERE的NULL值

Question

我无法使用WHEN和WHERE更新NULL行。我的查询出了什么问题？

UPDATE `product` SET `city` = CASE `city` 
WHEN NULL THEN 'Gangnam' 
WHEN 'Seouls' THEN 'Seoul' 
END 
WHERE city IN (NULL, 'Seoul') 

Answer 1

使用IS NULL比较NULL

UPDATE `product` 
SET `city` = 
    CASE  
      WHEN `city` IS NULL THEN 'Gangnam' 
      WHEN `city` = 'Seouls' THEN 'Seoul' 
    END 
WHERE city is NULL OR city = 'Seoul' 

Answer 2

在SQL的时候，用NULL一个comparaison永远是假的，所以你必须起诉特殊的运算符IS NULL

另一种方法是分裂你查询二：

UPDATE product SET city ='江南'WHERE city IS NULL

UPDATE product SET city = 'Seouls' WHERE city = '首尔'

Answer 3

UPDATE `product` SET `city` = 
CASE 
    WHEN `city` IS NULL THEN 'Gangnam' 
    WHEN `city` = 'Seouls' THEN 'Seoul' 
END 
WHERE city IS NULL OR city = 'Seouls' 

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