GROUP BY和內部聯接基於時間戳

Question

最新記錄我有一個History表如下：

ID | GroupCode | Category | TimeStamp 
---+-----------+----------+----------- 
1 | x   | shoes | 2016-09-01 
2 | y   | blach | 2016-09-01 

History表得到更新每個月和每個GroupCode單個條目被插入表中。

我也有一個Current表，其中包含最新的位置。

在我更新History表與當前位置之前或之後，我想知道Category是否已從上個月更改爲本月。

我需要比較最近的Category與當前的Category，如果它已更改，則在Current表中標記CategoryChanged。

Current表：

ID | GroupCode | Category | CategoryChanged 
---+-----------+----------+---------------- 
1 | x   | shoes | True  
2 | y   | blah  | False 

我試着用INNER JOIN來實現這一點，但我有在History表困難INNER JOIN到最近一個月和一年的條目，但沒有成功。下面

Answer 1

--get highest group code based on timestamp 
;with History 
    as 
    (select top 1 with ties groupcode,category 
    from 
    history 
    order by 
    row_number() over (partition by group code order by timestamp desc) as rownum 
    ) 

--now do a left join with current table 
    select 
    ct.ID, 
    ct.GroupCode, 
    ct.Category, 
    case when ct.category=ht.category or ht.category is null then 'False' 
     else 'true' 
    end as 'changed' 
    from 
    currenttable ct 
    left join 
    history ht 
    on ht.groupcode=ct.groupcode 

使用更新，檢查是否你的選擇值是正確的以後。

update ct 
set ct.category= 
case when ct.category=ht.category or ht.category is null then 'False' 
      else 'true' 
     end 
     from 
     currenttable ct 
     left join 
     history ht 
     on ht.groupcode=ct.groupcode 

Answer 2

，如果你犯了一個CTE其中歷史記錄具有rown_numbwer按日期排序從高到低每個GroupCode，那麼你有興趣的行1和2，所以你因此可以加入你的CTE上GroupCode，並選擇記錄1和2，您可以查看是否有類行1和2

;WITH CTE AS (SELECT *, row_number() OVER (PARTITION BY GroupCode ORDER BY TimeStamp Desc) RN FROM History) 
    SELECT 
      C1.ID, 
      C1.GroupCode, 
      C1.Category, 
      CASE WHEN C1.Category = C2.Category THEN 
       'false' 
      else 
       'true' 
      end AS CategoryChanged 

       FROM CTE C1 
         JOIN 
          CTE C2 
           ON C1.GroupCode = C2.GroupCode 
            AND C1.Rn=1 AND C2.RN = 2; 

之間變化如果你有空類別，你可以避免使用 - 順便說一句，你將需要學習如何處理你想要處理它們的NULL - 你不能指望人們在這裏發佈關於你永遠不會提到的NULL的問題！併發生意識到你想與他們做什麼

;WITH CTE AS (SELECT *, row_number() OVER (PARTITION BY GroupCode ORDER BY TimeStamp Desc) RN FROM History) 
    SELECT 
      C1.ID, 
      C1.GroupCode, 
      C1.Category, 
      CASE WHEN C1.Category = C2.Category OR C1.Category IS NULL AND C2.Category IS NULL THEN 
       'false' 
      else 
       'true' 
      end AS CategoryChanged 

       FROM CTE C1 
         JOIN 
          CTE C2 
           ON C1.GroupCode = C2.GroupCode 
            AND C1.Rn=1 AND C2.RN = 2;