UPDATE FROM in Azure SQL DW?

Question

I'm receiving an error in Azure SQL DW trying to do an UPDATE FROM query. The error is "FROM clause in UPDATE and DELETE statements cannot contain subquery sources or joins"

Is this just specific to SQL DW? I don't see anything wrong with this query otherwise. If it is a limitation of SQL DW, what's the alternative?

-- 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

As per the documentation Azure SQL Data Warehouse does support UPDATE but does not support ANSI joins in the FROM clause. You can use CTAS to work around. Simple two-table update:

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;

More complex example with CTAS, copied wholesale from the main UPDATE documentation page:

-- 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
;