如何在SQL中動態產生帶有計數的列進行資料探勘？

使用SQL動態產生欄位

本文探討資料探勘領域一個常見問題：基於動態資料動態建立列。當需要以使用者友善的格式呈現資料時，尤其是在每個動態產生的欄位中都需要值的計數時，就會出現這種挑戰。

問題陳述

我們有三個表：Customers、CustomerRewards和Rewards。目標是產生一個新表，該表顯示每個客戶的姓名以及他們在每種獎勵類型（例如，青銅、白銀、黃金等）中擁有的獎勵數量。但是，獎勵類型是動態的，這意味著隨著時間的推移，可以新增或刪除新的類型。

解：使用PIVOT函數

靜態PIVOT：

如果事先知道獎勵類型的數量，我們可以使用硬編碼的PIVOT函數。例如：

<code class="language-sql">select name, [Bronze], [Silver], [Gold], [Platinum], [AnotherOne]
from
(
  select c.name,
    cr.description,
    r.typeid
  from customers c
  left join rewards r
    on c.id = r.customerid
  left join customerrewards cr
    on r.typeid = cr.typeid
) x
pivot
(
  count(typeid)
  for description in ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])
) p;</code>

動態PIVOT：

如果獎勵類型的數量可能會有所不同，我們可以使用動態SQL來執行PIVOT：

<code class="language-sql">DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT ',' + QUOTENAME(description)
                    from customerrewards
                    group by description, typeid
                    order by typeid
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')

set @query = 'SELECT name,' + @cols + ' from
             (
                select c.name,
                  cr.description,
                  r.typeid
                from customers c
                left join rewards r
                  on c.id = r.customerid
                left join customerrewards cr
                  on r.typeid = cr.typeid
            ) x
            pivot
            (
                count(typeid)
                for description in (' + @cols + ')
            ) p '

execute(@query)</code>

包含總計欄位

要包含總計列，我們可以使用ROLLUP：

靜態ROLLUP：

<code class="language-sql">select name, sum([Bronze]) Bronze, sum([Silver]) Silver,
  sum([Gold]) Gold, sum([Platinum]) Platinum, sum([AnotherOne]) AnotherOne
from
(
  select name, [Bronze], [Silver], [Gold], [Platinum], [AnotherOne]
  from
  (
    select c.name,
      cr.description,
      r.typeid
    from customers c
    left join rewards r
      on c.id = r.customerid
    left join customerrewards cr
      on r.typeid = cr.typeid
  ) x
  pivot
  (
    count(typeid)
    for description in ([Bronze], [Silver], [Gold], [Platinum], [AnotherOne])
  ) p
) x
group by name with rollup</code>

動態ROLLUP：

<code class="language-sql">DECLARE @cols AS NVARCHAR(MAX),
    @colsRollup AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT ',' + QUOTENAME(description)
                    from customerrewards
                    group by description, typeid
                    order by typeid
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')

select @colsRollup
      = STUFF((SELECT ', Sum(' + QUOTENAME(description) + ') as ' + QUOTENAME(description)
                    from customerrewards
                    group by description, typeid
                    order by typeid
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')


set @query
          = 'SELECT name, ' + @colsRollup + '
             FROM
             (
                SELECT name,' + @cols + ' from
                 (
                    select c.name,
                      cr.description,
                      r.typeid
                    from customers c
                    left join rewards r
                      on c.id = r.customerid
                    left join customerrewards cr
                      on r.typeid = cr.typeid
                ) x
                pivot
                (
                    count(typeid)
                    for description in (' + @cols + ')
                ) p
              ) x1
              GROUP BY name with ROLLUP'

execute(@query)</code>

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