Merge pull request #197340 from lucaferrari77/patch-1

Update sql-data-warehouse-develop-ctas.md

Author: jborsecnik

articles/synapse-analytics/sql-data-warehouse/sql-data-warehouse-develop-ctas.md

@@ -1,20 +1,20 @@
 ---
 title: CREATE TABLE AS SELECT (CTAS) 
-description: Explanation and examples of the CREATE TABLE AS SELECT (CTAS) statement in Synapse SQL for developing solutions.
+description: Explanation and examples of the CREATE TABLE AS SELECT (CTAS) statement in dedicated SQL pool (formerly SQL DW) for developing solutions.
 author: joannapea
 manager: craigg
 ms.service: synapse-analytics
 ms.topic: conceptual
 ms.subservice: sql-dw 
-ms.date: 03/26/2019
+ms.date: 06/09/2022
 ms.author: joanpo
 ms.reviewer: wiassaf
 ms.custom: seoapril2019, azure-synapse
 ---
 
 # CREATE TABLE AS SELECT (CTAS)
 
-This article explains the CREATE TABLE AS SELECT (CTAS) T-SQL statement in Synapse SQL for developing solutions. The article also provides code examples.
+This article explains the CREATE TABLE AS SELECT (CTAS) T-SQL statement in dedicated SQL pool (formerly SQL DW) for developing solutions. The article also provides code examples.
 
 ## CREATE TABLE AS SELECT
 
@@ -55,7 +55,7 @@ FROM    [dbo].[FactInternetSales];
 
 Perhaps one of the most common uses of CTAS is creating a copy of a table in order to change the DDL. Let's say you originally created your table as `ROUND_ROBIN`, and now want to change it to a table distributed on a column. CTAS is how you would change the distribution column. You can also use CTAS to change partitioning, indexing, or column types.
 
-Let's say you created this table by using the default distribution type of `ROUND_ROBIN`, not specifying a distribution column in the `CREATE TABLE`.
+Let's say you created this table by specifying HEAP and using the default distribution type of `ROUND_ROBIN`.
 
 ```sql
 CREATE TABLE FactInternetSales
@@ -82,7 +82,12 @@ CREATE TABLE FactInternetSales
     TaxAmt money NOT NULL,
     Freight money NOT NULL,
     CarrierTrackingNumber nvarchar(25),
-    CustomerPONumber nvarchar(25));
+    CustomerPONumber nvarchar(25)
+)
+WITH( 
+ HEAP, 
+ DISTRIBUTION = ROUND_ROBIN 
+);
 ```
 
 Now you want to create a new copy of this table, with a `Clustered Columnstore Index`, so you can take advantage of the performance of Clustered Columnstore tables. You also want to distribute this table on `ProductKey`, because you're anticipating joins on this column and want to avoid data movement during joins on `ProductKey`. Lastly, you also want to add partitioning on `OrderDateKey`, so you can quickly delete old data by dropping old partitions. Here is the CTAS statement, which copies your old table into a new table.
@@ -115,116 +120,6 @@ RENAME OBJECT FactInternetSales_new TO FactInternetSales;
 DROP TABLE FactInternetSales_old;
 ```
 
-## Use CTAS to work around unsupported features
-
-You can also use CTAS to work around a number of the unsupported features listed below. This method can often prove helpful, because not only will your code be compliant, but it will often run faster on Synapse SQL. This performance is a result of its fully parallelized design. Scenarios include:
-
-* ANSI JOINS on UPDATEs
-* ANSI JOINs on DELETEs
-* MERGE statement
-
-> [!TIP]
-> Try to think "CTAS first." Solving a problem by using CTAS is generally a good approach, even if you're writing more data as a result.
-
-## ANSI join replacement for update statements
-
-You might find that you have a complex update. The update joins more than two tables together by using ANSI join syntax to perform the UPDATE or DELETE.
-
-Imagine you had to update this table:
-
-```sql
-CREATE TABLE [dbo].[AnnualCategorySales]
-( [EnglishProductCategoryName]    NVARCHAR(50)    NOT NULL
-, [CalendarYear]                    SMALLINT        NOT NULL
-, [TotalSalesAmount]                MONEY            NOT NULL
-)
-WITH
-(
-    DISTRIBUTION = ROUND_ROBIN
-);
-```
-
-The original query might have looked something like this example:
-
-```sql
-UPDATE    acs
-SET        [TotalSalesAmount] = [fis].[TotalSalesAmount]
-FROM    [dbo].[AnnualCategorySales]     AS acs
-JOIN    (
-        SELECT [EnglishProductCategoryName]
-        , [CalendarYear]
-        , SUM([SalesAmount])                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]
-        ) AS fis
-ON    [acs].[EnglishProductCategoryName]    = [fis].[EnglishProductCategoryName]
-AND    [acs].[CalendarYear]                = [fis].[CalendarYear];
-```
-
-Synapse SQL doesn't support ANSI joins in the `FROM` clause of an `UPDATE` statement, so you can't use the previous example without modifying it.
-
-You can use a combination of a CTAS and an implicit join to replace the previous example:
-
-```sql
--- 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;
-```
-
-## ANSI join replacement for MERGE 
-
-In Azure Synapse Analytics, [MERGE](/sql/t-sql/statements/merge-transact-sql?view=azure-sqldw-latest&preserve-view=true) (preview) with NOT MATCHED BY TARGET requires the target to be a HASH distributed table.  Users can use the ANSI JOIN with [UPDATE](/sql/t-sql/queries/update-transact-sql?view=azure-sqldw-latest&preserve-view=true) or [DELETE](/sql/t-sql/statements/delete-transact-sql?view=azure-sqldw-latest&preserve-view=true) as a workaround to modify target table data based on the result from joining with another table.  Here is an example.
-
-```sql
-CREATE TABLE dbo.Table1   
-    (ColA INT NOT NULL, ColB DECIMAL(10,3) NOT NULL);  
-GO  
-CREATE TABLE dbo.Table2   
-    (ColA INT NOT NULL, ColB DECIMAL(10,3) NOT NULL);  
-GO  
-INSERT INTO dbo.Table1 VALUES(1, 10.0);  
-INSERT INTO dbo.Table2 VALUES(1, 0.0);  
-GO  
-UPDATE dbo.Table2   
-SET dbo.Table2.ColB = dbo.Table2.ColB + dbo.Table1.ColB  
-FROM dbo.Table2   
-    INNER JOIN dbo.Table1   
-    ON (dbo.Table2.ColA = dbo.Table1.ColA);  
-GO  
-SELECT ColA, ColB   
-FROM dbo.Table2;
-
-```
-
 ## Explicitly state data type and nullability of output
 
 When migrating code, you might find you run across this type of coding pattern:
@@ -377,4 +272,4 @@ CTAS is one of the most important statements in Synapse SQL. Make sure you thoro
 
 ## Next steps
 
-For more development tips, see the [development overview](sql-data-warehouse-overview-develop.md).
+For more development tips, see the [development overview](sql-data-warehouse-overview-develop.md).