mysql - 是否可以将 2 个具有重复数据的 View 连接起来？

Question

我正在创建一个虚拟数据仓库并尝试加入 2 个单独的 View ，每个 View 都可以单独工作。但是，当组合起来时，我的 group by 语句中似乎存在某些内容，导致数据不正确。

create view CentreSizing as
select CentreID, Name,
case
  when Capacity between 0 and 199 then 'Small'
  when Capacity between 200 and 900 then 'Medium'
  else 'Large'
end as Size
from Centre;

create view sizeQuantities as
select cs.Size, count(*) as "Occurences" from
CentreSizing cs
group by cs.Size;

create view AvgSpendings2018 as
select cs.Size, monthname(d.DateStamp) as "Month", sum(e.Cost)/quant.Occurences as 
"AverageExpense"
from Expense e join CentreSizing cs on e.CentreID = cs.CentreID
join DateTime d on e.DateTimeID = d.DateTimeID
join sizeQuantities quant on quant.Size = cs.Size
where YEAR(d.DateStamp) = 2018
group by cs.Size, MONTH(d.DateStamp);

create view TotalSpendings2018 as
select e.CentreID, c.Name, cs.Size , monthname(d.DateStamp) as "Month", sum(e.Cost) as 
"TotalExpense" from Expense e
 left join DateTime d on e.DateTimeID = d.DateTimeID
 join Centre c on c.CentreID = e.CentreID
join CentreSizing cs on c.CentreID = cs.CentreID
where year(d.DateStamp) = 2018
group by month(d.DateStamp), e.CentreID;

所有 View 创建都按预期工作，但是当我尝试合并结果以显示每月总费用时，与类似规模的中心的平均费用相比，结果并不如预期。

select ts.CentreID, ts.Name, ts.Size, ts.Month, avg.AverageExpense
from TotalSpendings2018 ts join AvgSpendings2018 avg on
ts.Size = avg.Size
Group by ts.CentreID, ts.Size, ts.Month;

| CentreID | Name                               | Size   | Month     | AverageExpense 
|
+----------+-------------------------------------+--------+-----------+---------------- 
+
|        1 | Queen Alexandra Hospital           | Large  | April     |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | August    |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | December  |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | February  |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | January   |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | July      |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | June      |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | March     |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | May       |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | November  |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | October   |      1490.0000 
|
|        1 | Queen Alexandra Hospital           | Large  | September |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | April     |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | August    |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | December  |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | February  |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | January   |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | July      |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | June      |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | March     |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | May       |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | November  |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | October   |      1490.0000 
|
|        2 | Southampton General Hospital       | Large  | September |      1490.0000 
|
|        3 | St Richard's Hospital              | Medium | April     |       445.0000 
|
|        3 | St Richard's Hospital              | Medium | August    |       445.0000 
|
|        3 | St Richard's Hospital              | Medium | December  |       445.0000 
|

Answer 1

查看 TotalSpendings2018 在 GROUP BY 子句中缺少 c.Name(如果每个中心都有唯一的名称，那应该没问题)。虽然与大多数其他 RDBMS 不同，mysql 允许这样做，但这很容易出错并且不是良好的编码实践。

关于查询:

• 在我看来，您实际上不需要 GROUP BY，因为没有使用聚合函数(否则:GROUP BY 中缺少 ts.Name 和 avg.AverageExpense)
• 根据 View 的描述，我还猜测您缺少“月份”的联接条件，因为此列在两个 View 中都可用
• 为什么不在输出中显示 TotalExpense？我本以为这就是您想要做的(否则，您不需要加入)

我(希望受过教育)对您的查询的猜测:

select 
    ts.CentreID,
    ts.Name, 
    ts.Size, 
    ts.Month, 
    avg.AverageExpense,
    ts.TotalExpense
from
    TotalSpendings2018 ts 
    join AvgSpendings2018 avg 
    on ts.Size = avg.Size
    and ts.Month = avg.Month