MySQL 错误 : column cannot be null in update query

Question

我有一个运行良好的 SQL 更新查询。

Update inventory set reserved = reserved - 
CASE 
    WHEN location_id = 23 and variant_id = 40 then 2
    WHEN location_id = 13 and variant_id = 20 then 3
end 
where (location_id = 23 and variant_id = 40) or 
      (location_id = 13 and variant_id = 20);

问题是我想检查更新，以便该值不会低于零。我想在 case when 这样的语句中加入一个额外的条件

WHEN location_id = 13 and variant_id = 20 and reserved > 0 then 3

但它会抛出错误“保留的列不能为空”。我该如何解决这个问题？

Answer 1

因为 CASE 表达式中的条件与 WHERE 子句中的条件不匹配，所以您的 需要一个 ELSE 子句>CASE 表达式，以便当 WHEN 条件都不为真时，您不会返回 NULL 值，这将导致

reserved - (CASE ...) 

为 NULL，给出您所看到的错误。试试这个:

Update inventory set reserved = reserved - 
CASE 
    WHEN location_id = 23 and variant_id = 40 then 2
    WHEN location_id = 13 and variant_id = 20 and reserved > 0 then 3
    ELSE 0
end 
where (location_id = 23 and variant_id = 40) or 
      (location_id = 13 and variant_id = 20);

或者，为了防止值低于 0，您可能需要考虑使用 LEAST 来防止减去大于当前 reserved 值的值，例如

Update inventory set reserved = reserved - 
CASE 
    WHEN location_id = 23 and variant_id = 40 then LEAST(2, reserved)
    WHEN location_id = 13 and variant_id = 20 then LEAST(3, reserved)
end 
where (location_id = 23 and variant_id = 40) or 
      (location_id = 13 and variant_id = 20);