從 Azure SQL DW 中更新？

Question

我在嘗試執行 UPDATE FROM 查詢時在 Azure SQL DW 中收到錯誤。 錯誤是“UPDATE 和 DELETE 語句中的 FROM 子句不能包含子查詢源或連接”

這只是特定於 SQL DW 嗎？ 否則，我認為此查詢沒有任何問題。 如果是 SQL DW 的限制，有什么替代方案？

-- Permanent fact table with 5 billion rows
CREATE TABLE FactTable (Id1 INT, Id2 INT, EmailAddress NVARCHAR(100), Value1 INT)
WITH (DISTRIBUTION = HASH(EmailAddress));

-- Staging fact table with 10 million rows    
CREATE TABLE StageTable (Id1 INT, Id2 INT, EmailAddress NVARCHAR(100), Value1 INT)
WITH (DISTRIBUTION = HASH(EmailAddress), HEAP);

-- Add a secondary index that should help with joining to StageTable
CREATE NONCLUSTERED INDEX ix ON FactTable (Id1, Id2);

UPDATE fact
SET
   Value1 = CASE WHEN stage.Value1 > fact.Value1 THEN stage.Value1 ELSE fact.Value1 END
FROM FactTable AS fact
INNER JOIN StageTable AS stage ON fact.Id1 = stage.Id1 AND fact.Id2 = stage.Id2

Answer 1

根據文檔Azure SQL 數據倉庫確實支持UPDATE但不支持FROM子句中的 ANSI 連接。 您可以使用 CTAS 來解決。 簡單的兩表更新：

UPDATE dbo.FactTable
SET
   Value1 = CASE WHEN stage.Value1 > dbo.FactTable.Value1 THEN stage.Value1 ELSE dbo.FactTable.Value1 END
FROM dbo.StageTable AS stage
WHERE dbo.FactTable.Id1 = stage.Id1 
  AND dbo.FactTable.Id2 = stage.Id2;

更復雜的 CTAS 示例，從主更新文檔頁面批發復制：

-- Create an interim table
CREATE TABLE CTAS_acs
WITH (DISTRIBUTION = ROUND_ROBIN)
AS
SELECT  ISNULL(CAST([EnglishProductCategoryName] AS NVARCHAR(50)),0)    AS [EnglishProductCategoryName]
,       ISNULL(CAST([CalendarYear] AS SMALLINT),0)                      AS [CalendarYear]
,       ISNULL(CAST(SUM([SalesAmount]) AS MONEY),0)                     AS [TotalSalesAmount]
FROM    [dbo].[FactInternetSales]       AS s
JOIN    [dbo].[DimDate]                 AS d    ON s.[OrderDateKey]             = d.[DateKey]
JOIN    [dbo].[DimProduct]              AS p    ON s.[ProductKey]               = p.[ProductKey]
JOIN    [dbo].[DimProductSubCategory]   AS u    ON p.[ProductSubcategoryKey]    = u.[ProductSubcategoryKey]
JOIN    [dbo].[DimProductCategory]      AS c    ON u.[ProductCategoryKey]       = c.[ProductCategoryKey]
WHERE   [CalendarYear] = 2004
GROUP BY
        [EnglishProductCategoryName]
,       [CalendarYear]
;

-- Use an implicit join to perform the update
UPDATE  AnnualCategorySales
SET     AnnualCategorySales.TotalSalesAmount = CTAS_ACS.TotalSalesAmount
FROM    CTAS_acs
WHERE   CTAS_acs.[EnglishProductCategoryName] = AnnualCategorySales.[EnglishProductCategoryName]
AND     CTAS_acs.[CalendarYear]               = AnnualCategorySales.[CalendarYear]
;

--Drop the interim table
DROP TABLE CTAS_acs
;

Answer 2

我發現使用 ASDW（和 APS/PDW）避免像瘟疫一樣的批量更新是一個很好的做法。

這是一個純粹的 CTAS 替代方案，在您更新大量行的情況下會更快。

它假設 id1 是一個相對較好的分布鍵，並且暫存行數少於事實行數，從而使復制可行。 此策略應消除節點之間的數據移動。

如果您有一個非常大的臨時表，在每個表中創建一個由 id1 和 id2 組合而成的代理列，然后通過該列的散列分布兩個表，將提供更好的性能。

create  table FactTable (
        id1 int,
        id2 int,
        value1 int)
with    (distribution = hash(id1));

create  table StageTable (
        id1 int,
        id2 int,
        value1 int)
with    (distribution = replicate);

create  table UpdatedFact 
with    (distribution = hash(id1)) 
as
select  f.id1,
        f.id2,
        case when s.id1 is not null and s.value1 > f.value1 
            then s.value1
            else f.value1
            end as value1
from    FactTable f
        left outer join StageTable s
        on s.id1 = f.id1
        and s.id2 = f.id2

truncate table FactTable;
alter table UpdatedFact switch to FactTable;
drop table UpdatedFact;

Answer 3

簡化你的嘗試是可行的。 只需去掉連接並從另一個表更新一個表。

update FactTable
set this = that
from StageTable s where s.something = FactTable.something

這是否是最佳方法取決於您的情況，但它會執行而不會引發錯誤。