首页 \ 问答 \ 按“每30天”分组mysql结果(Group mysql results by “every 30 days”)

按“每30天”分组mysql结果(Group mysql results by “every 30 days”)

 我有一个问题：  
SELECT 
  COUNT(id) as amount,
  DATEDIFF(expire, buydate) as days
FROM `vw8mv_orders`
GROUP BY MONTH(expire)
 
 结果是：  
    amount  days
    1       22
    1       30
    1       105
    1       161
 
 我想在一组（每30天）看到这些结果。 如果天数值在1到30天之间，则将其放入30天组，如果下注31-60，则放入60天组等。  
 例如：  
    amount  time
      2     30 days
      0     60 days
      1     90 days

I have a query: 
SELECT 
  COUNT(id) as amount,
  DATEDIFF(expire, buydate) as days
FROM `vw8mv_orders`
GROUP BY MONTH(expire)
 
The result is: 
    amount  days
    1       22
    1       30
    1       105
    1       161
 
I'd like to see these results in a group (every 30 days). If days value is between 1 and 30 days, then put this in 30days group, if bet 31-60, put to 60days group, etc. 
For example: 
    amount  time
      2     30 days
      0     60 days
      1     90 days

原文：https://stackoverflow.com/questions/23545695

更新时间：2022-02-09 06:02

最满意答案

 
  
   我的困惑是，我们如何不必明确初始化矢量  
  
 
 这是std :: unordered_map :: operator []的预期行为，如果该键不存在，将执行带有值初始化映射值的插入。  
 
  返回映射到与键等效的键的值的引用，如果此键尚不存在，则执行插入操作。  
 
 这意味着对于hashtable[s].push_back("bar"); ，首先插入一个值初始化的std::vector （即一个空的std::vector ），然后由std::unordered_map::operator[]引用返回该向量。 然后在vector上调用push_back("bar") （然后它的大小变为1并包含一个元素）。  
 
  
   是不是将“bar”推入带有垃圾值的向量？  
  
 
 不， std::vector与原始数组不同，它的大小是动态的。 如上所述，值初始化的std::vector为空，其大小为0 ，仍然不包含任何元素（以及任何“垃圾值”）。 

 
  
  My confusion is how come we don't have to explicitly initialize a vector 
  
 
This is the expected behavior of std::unordered_map::operator[], which will perform an insertion with value-initialized mapped value if the key does not exist. 
 
 Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist. 
 
That means for hashtable[s].push_back("bar");, a value-initialized std::vector (i.e. an empty std::vector) will be inserted at first, then the vector will be returned by reference by std::unordered_map::operator[]. Then push_back("bar") is called on the vector (then its size becomes 1 and contains one element). 
 
  
  isn't pushing "bar" into a vector with garbage values? 
  
 
No, std::vector is not same as raw array, its size is dynamic. As explained above, the value-initialized std::vector is empty, its size is 0, still doesn't contain any elements (and any "garbage values").

按“每30天”分组mysql结果(Group mysql results by “every 30 days”)

最满意答案

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