Question

我正在构建一个MySQL查询，以确定在给定日期范围内出现多个类别中的每个项目的数量。我最初的尝试看起来像这样：

select Title, 
  (select count(*) from entries where CategoryID=1
    and Date >= @StartDate and Date <= @EndDate) as Cat1,
  (select count(*) from entries where CategoryID=2
   and Date >= @StartDate and Date <= @EndDate) as Cat2,
  (select count(*) from entries where CategoryID is null
   and Date >= @StartDate and Date <= @EndDate) as UnkownCategory
from entries
  where Date >= @StartDate and Date <= @EndDate

该表非常大，我想重构查询以加快速度，但我不确定如何使用GROUP BY / HAVING语句重写 - 还是我还缺少另一种方法？

修改：示例结果集 - 如下所示：

Title | Category 1 Total | Category 2 Total | Unknown Category Total
ABC     1                  3                  0
DEF     2                  7                  2

Answer 1

select Title, SUM(CategoryID=1) as Cat1, SUM(categoryID=2) as Cat2,
SUM(categoryID IS NULL) as UnknownCategory
FROM entries
WHERE Date BETWEEN @StartDate AND @EndDate
GROUP BY Title

你可以在sum（）函数中粘贴表达式：真值等于1，false等于0.我也使用了BETWEEN运算符，这个运算符要快一点。

一种可以返回不同结果布局但在概念上更简单的替代方案：

select Title, CategoryID, count(*)
from entries
WHERE Date BETWEEN @StartDate AND @EndDate
group by Title, CategoryID

Answer 2

Select COUNT(*), sTitle, CategoryID FROM entries 
WHERE Date >= @StartDate and Date <= @EndDate 
GROUP BY CategoryID, sTitle

Answer 3

如何按类别ID进行分组，然后使用having语句过滤掉特定类别，例如：

select CategoryID, count(*) 
from entries 
where Date >= @StartDate AND Date <= @EndDate
group by CategoryID
having CategoryID = 1 or CategoryID = 2 or CategoryID is null

如果每个类别有多个标题，您可以按两个字段进行分组：

select Title, CategoryID, count(*) 
from entries 
where Date >= @StartDate AND Date <= @EndDate
group by Title, CategoryID
having CategoryID = 1 or CategoryID = 2 or CategoryID is null

使用GROUP BY / HAVING重构子查询？

3 个答案: