Category: SQL Server

In my previous post, I provided the design pattern and Biml for a pure Type 2 Slowly Changing Dimension (SCD). When I say “pure Type 2 SCD”, I mean an ETL process that adds a new row for a change in any field in the dimension and never updates a dimension attribute without creating a new row.  In practice, I tend to create more hybrid Type 2 SCDs where updates to some attributes require a new row and others update the value on the existing rows. A similar pattern that I find I implement more often than a pure Type 2 is a Type 6 SCD. A Type 6 SCD builds on the Type 2 technique by adding current attributes alongside the historical attributes so related measures can be grouped by the historical or current dimension attribute values. The only difference between what I call a hybrid Type 2 and a Type 6 is that in the Type 6, there are no Type 1 attributes in the dimension that do not also have a Type 2 version in the dimension to capture the historical values.

Design Pattern

Control Flow

If you are comfortable with my design pattern for a pure Type 2 SCD in which a change of value in any column causes a new row, this pattern is quite similar. And the control flow is exactly the same. This pattern, as with my pure Type 2, assumes that rows are not deleted from the source system. You could easily modify this to check for deleted rows if needed.

The steps in the control flow are:

1. Log package start.
2. Truncate the update table.
3. Execute the data flow.
4. Execute the update statements to update columns and insert new rows.
5. Log package end.

The update statements are different in this pattern, and I’ll explain those in detail below.

Data Flow

The data flow looks like a pure Type 2 SCD, with the exception of an added derived column transformation and minor changes to the lookup and conditional split. Again, I use views to integrate the data, apply business logic, and add hashkeys for change detection. Then I use SSIS to perform the mechanics of loading the data.

The steps in this data flow task are:

1. Retrieve data from my source view.
2. Count the rows for package logging purposes.
3. Perform a lookup to see if the entity already exists in the dimension table.
4. If the entity doesn’t exist at all in the dimension table, it goes into the left path where I count the number of rows, add a derived column that sets the row start date to “01/01/1900 00:00:00”, and then insert the row into the dimension table.
5. If the entity does exist in the table, I check it for changes.
6. If there are changes to the entity, I count the number of rows, us a derived column to flag the type(s) of changes to make, and then insert the row into an update table.
7. Entities with no changes are simply counted for audit purposes.

The Source View

This SSIS pattern requires 3 hashed values for for change detection:

• HistoricalHashKey: the unique identifier of the entity, the natural key that ties the historical rows together
• ChangeHashKey: the columns on the dimension that cause a new row to be created and the current row to be expired
• UpdateHashKey: the columns on the dimension that should be updated in place

In my example view below, the Route ID and Warehouse identify a unique route.  The supervisor, route description and route type are all important attributes of the route.  The route area identifies the metro area in which a route is located. If this should change, there is no need to see previous values; we just want to see the current value on every row.

 CREATE View [dbo].[StgDimRoute] as   
    with [Routebase] as   
    (   SELECT [RouteID]  
    , [RouteDescription]  
    , [Supervisor]  
    , [RouteType]  
    , [Warehouse]  
    , [RouteArea]  
     , 1 as RowIsCurrent     
  FROM Stg.Routes R  
  Left JOIN dbo.StgWarehouse W ON W.WarehouseID = R.WarehouseID  
  UNION   
  select -1 as RouteID, 'Unknown' as RouteDescription, 'Unknown' as Supervisor,   
  'Unknown' as RouteType, 'Unknown' as Warehouse, 'Unknown' as RouteArea, 
  1 as RowIsCurrent 
      ) ,  
 Routedata as   
 (  
 Select RouteID, RouteDescription, Supervisor, RouteType, Warehouse, 
   RouteArea, RowIsCurrent 
  , CONVERT(VARBINARY(20), HASHBYTES('MD5', CONCAT([RouteID], ' ',Warehouse ))) 
      AS HistoricalHashKey  
  , CONVERT(VARBINARY(20), HASHBYTES('MD5', CONCAT(Supervisor, [RouteDescription], 
      RouteType))) AS ChangeHashKey  
  , Convert(VARBINARY(20), HASHBYTES('MD5', CONCAT(Area,' '))) AS UpdateHashKey  
  from Routebase sb  
  )   
 Select RouteID, RouteDescription, Supervisor, RouteType, Warehouse,  
  RouteArea, RowIsCurrent  
  , HistoricalHashKey, ChangeHashKey, UpdateHashKey  
  , CONVERT(VARCHAR(34), HistoricalHashKey, 1) AS HistoricalHashKeyASCII  
  , CONVERT(VARCHAR(34), ChangeHashKey, 1) AS ChangeHashKeyASCII  
  , CONVERT(VARCHAR(34), UpdateHashKey, 1) AS UpdateHashKeyASCII  
  , dateadd(MINUTE, -1, current_timestamp) as RowEndDate
  , CURRENT_TIMESTAMP as RowStartDate   
  from Routedata ds  

The RowEndDate value in this view will be used for routes that require a current row to be expired since my pattern is the leave the end date of the current row null.

The Change Detection Lookup

The lookup in my DFT compares the HistoricalHashKeyASCII column from the source view with the varchar version of the HistoricalHashKey from the dimension table and adds two lookup columns: lkp_ChangeHashKeyASCII and lkp_UpdateHashKeyASCII to the data set.

Rows that do not match are new rows; i.e., that route has never been in the dimension table before.  Rows that do match have a row in the dimension table and will then be evaluated to see if there are any changes in the values for that route.

Derived Column for New Rows

The no match output of the lookup are new rows for routes that are not in the dimension table. Since this is the first row in the table for that route, we want this row to be effective from the beginning of time until the end of time. The beginning of time in this data mart is 01/01/1900. Since the data is loaded multiple times per day, I use a date/time field rather than a date. If you only need the precision of a day, you can cut your row end date/time back to just a date. In my pattern, the current row has a null row end date, but you could easily add a derived column to set the end date to 12/31/9999 if you prefer.

Conditional Split for Change Detection

This time, I have to check to see if both the ChangeHashKeyASCII and the UpdateHashKeyASCII match in my conditional split.

If both hashed values from the source view match the hashed values from the dimension table, then no changes are required and the row is simply counted for audit purposes.

If either hashed value doesn’t match, there is an update to be made.

Derived Column to Identify Change Types

We again compare the UpdateHashKeyASCII value from the source view with that of the dimension. If they don’t match, we set the UpdateInPlace flag to true. If the ChangeHashKeyASCII values don’t match, we set the UpdateNewRow flag to true. If a row has both types of changes, both types of updates will be made.

My update table contains the UpdateInPlace and UpdateNewRow columns, so I can reference these flags in my update statements.

The Update Statements

The update statements in the control flow take the changes from the update table and apply them to the dimension table. Three statements are executed in the Execute SQL Statement labeled SQL Update DimRoute.

 UPDATE A   
 SET  A.RouteArea = UA.RouteArea,   
      A.UpdateHashKey = UA.UpdateHashKey,   
      A.UpdtDtTm = CURRENT_TIMESTAMP   
 FROM  Updt.UpdtRoute AS UA   
     JOIN Dbo.DimRoute AS A   
      ON UA.HistoricalHashKey = A.HistoricalHashKey   
 WHERE UpdateInPlace = 1; 
  
UPDATE A   
 SET  RowIsCurrent = 0,   
     A.RowEndDate = UA.RowEndDate,   
     A.UpdtDtTm = CURRENT_TIMESTAMP   
 FROM  Updt.UpdtRoute UA   
     JOIN Dbo.DimRoute AS A   
      ON UA.HistoricalHashKey = A.HistoricalHashKey   
 WHERE A.RowIsCurrent = 1   
     AND UA.UpdateNewRow = 1;   

 INSERT INTO Dbo.DimRoute   
       (RouteID,   
        RouteDescription,   
        Supervisor,   
        RouteType,   
        Warehouse,   
        RouteArea,   
        RowIsCurrent,   
        RowStartDate,   
        RowEndDate,   
        HistoricalHashKey,   
        ChangeHashKey,   
        UpdateHashKey)   
 SELECT RouteID,   
        RouteDescription,   
        Supervisor,   
        RouteType,   
        Warehouse,   
        RouteArea,   
        1,   
        RowStartDate,   
        null,   
        HistoricalHashKey,   
        ChangeHashKey,   
        UpdateHashKey
FROM Updt.UpdtRoute WHERE UpdateNewRow = 1; 

The first statement updates the values for the columns designated to be updated in place by joining the update table to the dimension table based on the HistoricalHashKey column. This is the same as performing updates in a Type 1 SCD.

The second statement expires all the rows for which a new row will be added. The third statement inserts the new rows with the RowIsCurrent value set to 1 and the RowEndDate set to null.

The Biml

If you are using Biml, you know that you can create a design pattern for this type of dimension load and reuse it across multiple projects. This speeds up development and ensures that your Type 2 Hybrid or Type 6 dimensions are implemented consistently.

As usual, I have 3 Biml files that are used to create the SSIS package:

• ProjectConnections.biml – contains all the project-level connections for the project
• Dim2Hybrid.biml – contains the SSIS design pattern with code nuggets that parameterize it to make it reusable
• CreateDim2HybridPackages.biml – calls Dim2Hybrid.biml and passes along the values to be used for each package

I’ve pasted Dim2Hybrid and CreateDim2Hybrid below.

Dim2Hybrid.Biml

 <#@ template language="C#" tier="2" #>  
 <#@ property name="PackageName" type="String" #>  
 <#@ property name="DstSchemaName" type="String" #>  
 <#@ property name="DstTableName" type="String" #>  
 <#@ property name="DstConnection" type="String" #>  
 <#@ property name="DataFlowSourceName" type="String" #>  
 <#@ property name="SrcConnection" type="String" #>  
 <#@ property name="SourceQuery" type="String" #>     
 <#@ property name="UpdateSchemaName" type="String" #>     
 <#@ property name="UpdateTableName" type="String" #>     
 <#@ property name="UpdateConnection" type="String" #>        
 <#@ property name="UpdateSQLStatement" type="String" #>      
     <Package Name="<#=PackageName#>" Language="None">  
   <Parameters>  
     <Parameter DataType="String" Name="ParentPackageID">00000000-0000-0000-0000-000000000000</Parameter>  
   </Parameters>  
   <Variables>  
     <Variable EvaluateAsExpression="true" DataType="String" IncludeInDebugDump="Exclude" Name="QualifiedTableSchema">"[" + @[User::SchemaName] + "].[" + @[User::TableName] + "]"</Variable>  
     <Variable DataType="String" IncludeInDebugDump="Exclude" Name="QueryAuditStart">EXECUTE [dbo].[PackageControlStart] @PackageName = ?, @PackageId = ?, @ParentPackageId = ?, @ExecutionId = ?, @StartTime = ?;</Variable>  
     <Variable DataType="String" IncludeInDebugDump="Exclude" Name="QueryAuditUpdate">EXECUTE [dbo].[PackageControlStop] @PackageId = ?, @ExecutionId = ?, @InsertRowQuantity = ?, @UpdateRowQuantity = ?, @UnchangedRowQuantity=?;</Variable>  
     <Variable DataType="Int32" IncludeInDebugDump="Include" Name="RowCountChanged">0</Variable>  
     <Variable DataType="Int32" IncludeInDebugDump="Include" Name="RowCountNew">0</Variable>  
     <Variable DataType="Int32" IncludeInDebugDump="Include" Name="RowCountSource">0</Variable>  
     <Variable DataType="Int32" IncludeInDebugDump="Include" Name="RowCountUnchanged">0</Variable>   
     <Variable DataType="String" IncludeInDebugDump="Exclude" Name="SchemaName"><#=DstSchemaName#></Variable>  
     <Variable DataType="String" IncludeInDebugDump="Exclude" Name="TableName"><#=DstTableName#></Variable>  
   </Variables>  
   <Tasks>    
     <Dataflow Name="DFT_Insert<#=DstTableName#>">  
       <Transformations>  
         <RowCount Name="CNT_Changed_Rows" VariableName="User.RowCountChanged">  
           <InputPath OutputPathName="CSPL Check For Changes.ChangedRows" />  
     </RowCount>  
         <ConditionalSplit Name="CSPL Check For Changes">  
           <InputPath OutputPathName="LKP Historical Key.Match" />  
           <OutputPaths>  
             <OutputPath Name="ChangedRows">  
               <Expression>(ChangeHashKeyASCII != lkp_ChangeHashKeyASCII) || (UpdateHashKeyASCII != lkp_UpdateHashKeyASCII)</Expression>  
     </OutputPath>  
   </OutputPaths>  
     </ConditionalSplit>  
         <RowCount Name="CNT_New_Rows" VariableName="User.RowCountNew">  
           <InputPath OutputPathName="LKP Historical Key.NoMatch" />  
     </RowCount>  
       <DerivedColumns Name="Identify Change Types">  
           <InputPath SsisName="Derived Column Input" OutputPathName="CNT_Changed_Rows.Output" />  
           <Columns>  
             <Column Name="UpdateInPlace" DataType="Boolean">UpdateHashKeyASCII != lkp_UpdateHashKeyASCII ? (DT_BOOL)1 : (DT_BOOL)0</Column>  
             <Column Name="UpdateNewRow" DataType="Boolean">ChangeHashKeyASCII != lkp_ChangeHashKeyASCII ? (DT_BOOL)1 : (DT_BOOL)0</Column>  
   </Columns>  
     </DerivedColumns>  
         <Lookup Name="LKP Historical Key" NoMatchBehavior="RedirectRowsToNoMatchOutput" OleDbConnectionName="<#=DstConnection#>">  
             <DirectInput>SELECT  
   CONVERT(VARCHAR(34), ChangeHashKey, 1) AS ChangeHashKeyASCII  
 , CONVERT(VARCHAR(34), HistoricalHashKey, 1) AS HistoricalHashKeyASCII  
  ,CONVERT(VARCHAR(34), UpdateHashKey, 1) as UpdateHashkeyAscii  
 FROM  
  <#=DstSchemaName#>.<#=DstTableName#>  
     where rowiscurrent = 1  
  </DirectInput> 
           <Parameters>  
             <Parameter SourceColumn="HistoricalHashKeyASCII" />  
   </Parameters>  
            <ParameterizedQuery>select * from (SELECT  
  CONVERT(VARCHAR(34), ChangeHashKey, 1) AS ChangeHashKeyASCII  
 , CONVERT(VARCHAR(34), HistoricalHashKey, 1) AS HistoricalHashKeyASCII  
  ,CONVERT(VARCHAR(34), UpdateHashKey, 1) as UpdateHashkeyAscii  
 FROM  
  <#=DstSchemaName#>.<#=DstTableName#>  
     where rowiscurrent = 1) [refTable]  
 where [refTable].[HistoricalHashKeyASCII] = ?</ParameterizedQuery>  
            <InputPath OutputPathName="CNT_Source_Rows.Output" />  
           <Inputs>  
             <Column SourceColumn="HistoricalHashKeyASCII" TargetColumn="HistoricalHashKeyASCII" />  
   </Inputs>  
           <Outputs>  
             <Column SourceColumn="ChangeHashKeyASCII" TargetColumn="lkp_ChangeHashKeyASCII" />  
                   <Column SourceColumn="UpdateHashkeyAscii" TargetColumn="lkp_UpdateHashKeyASCII" />  
   </Outputs>  
     </Lookup>  
         <DerivedColumns Name="NewItemRowStartDate">  
        <InputPath OutputPathName="CNT_New_Rows.Output" />  
          <Columns>  
         <Column ReplaceExisting="true" Name="RowStartDate" DataType="DateTime">  
          (DT_DBTIMESTAMP)"01/01/1900 00:00:00"  
         </Column>  
        </Columns>  
       </DerivedColumns>  
            <OleDbDestination Name="OLE_DST_New_Rows" ConnectionName="<#=DstConnection#>">  
           <InputPath OutputPathName="NewItemRowStartDate.Output" />  
            <ExternalTableOutput Table="<#=DstSchemaName#>.<#=DstTableName#>" />   
                <Columns>  
                   <Column SourceColumn="RowEndDate" IsUsed="false"/>  
                </Columns>  
     </OleDbDestination>  
         <RowCount Name="CNT_Source_Rows" VariableName="User.RowCountSource">  
           <InputPath OutputPathName="<#=DataFlowSourceName#>.Output" />  
     </RowCount>  
         <OleDbSource Name="<#=DataFlowSourceName#>" ConnectionName="<#=SrcConnection#>"> 
                <DirectInput><#=SourceQuery#></DirectInput>  
     </OleDbSource>  
         <RowCount Name="CNT_Unchanged_Rows" VariableName="User.RowCountUnchanged">  
           <InputPath OutputPathName="CSPL Check For Changes.Default" />  
     </RowCount>  
         <OleDbDestination Name="OLE_DST Update Table" ConnectionName="<#=DstConnection#>">   
           <InputPath OutputPathName="Identify Change Types.Output" />  
            <ExternalTableOutput Table="[<#=UpdateSchemaName#>].[<#=UpdateTableName#>]" />  
     </OleDbDestination>  
   </Transformations>  
        <PrecedenceConstraints>  
           <Inputs>  
              <Input OutputPathName="SQL Truncate <#=UpdateTableName#>.Output" />   
           </Inputs>  
        </PrecedenceConstraints>  
     </Dataflow>  
     <ExecuteSQL Name="SQL Begin Audit" ConnectionName="Audit">  
       <VariableInput VariableName="User.QueryAuditStart" />  
       <Parameters>  
         <Parameter Name="0" VariableName="System.PackageName" DataType="String" Length="-1" />  
         <Parameter Name="1" VariableName="System.PackageID" DataType="Guid" Length="-1" />  
         <Parameter Name="2" VariableName="ParentPackageID" DataType="Guid" Length="-1" />  
         <Parameter Name="3" VariableName="System.ServerExecutionID" DataType="Int64" Length="-1" />  
         <Parameter Name="4" VariableName="System.StartTime" DataType="Date" Length="-1" />  
   </Parameters>  
     </ExecuteSQL>  
     <ExecuteSQL Name="SQL Close Audit" ConnectionName="Audit">  
       <VariableInput VariableName="User.QueryAuditUpdate" />  
       <Parameters>  
         <Parameter Name="0" VariableName="System.PackageID" DataType="Guid" Length="-1" />  
         <Parameter Name="1" VariableName="System.ServerExecutionID" DataType="Int64" Length="-1" />  
         <Parameter Name="2" VariableName="User.RowCountNew" DataType="Int32" Length="-1" />  
         <Parameter Name="3" VariableName="User.RowCountChanged" DataType="Int32" Length="-1" />  
         <Parameter Name="4" VariableName="User.RowCountUnchanged" DataType="Int32" Length="-1" />  
   </Parameters>  
        <PrecedenceConstraints>  
           <Inputs>  
              <Input OutputPathName="SQL Update <#=DstTableName#>.Output" />  
           </Inputs>  
        </PrecedenceConstraints>  
     </ExecuteSQL>    
     <ExecuteSQL Name="SQL Truncate <#=UpdateTableName#>" ConnectionName="<#=UpdateConnection#>">  
       <DirectInput>Truncate Table [<#=UpdateSchemaName#>].[<#=UpdateTableName#>]</DirectInput>  
        <PrecedenceConstraints>  
           <Inputs>  
              <Input OutputPathName="SQL Begin Audit.Output" />  
           </Inputs>  
        </PrecedenceConstraints>  
     </ExecuteSQL>  
     <ExecuteSQL Name="SQL Update <#=DstTableName#>" ConnectionName="<#=DstConnection#>">  
          <DirectInput><#=UpdateSQLStatement#></DirectInput>  
        <PrecedenceConstraints>  
           <Inputs>  
              <Input OutputPathName="DFT_Insert<#=DstTableName#>.Output" /> 
           </Inputs>  
        </PrecedenceConstraints>  
     </ExecuteSQL>  
   </Tasks>  
 </Package>  

Create Dim2Hybrid.Biml

 <#@ template language="C#" hostspecific="true" #>  
 <Biml xmlns="http://schemas.varigence.com/biml.xsd">  
 <#@ include file="ProjectConnection.biml" #>  
 <!--  
    <#  
    string PackageName = "LoadDimRoute";  
    string DstSchemaName = "dbo";  
   string DstTableName = "DimRoute";  
    string DstConnection = "AWBIML";     
    string DataFlowSourceName = "OLE_SRC_StgDimRoute";  
    string SrcConnection = "AWBIML";  
    string SourceQuery = @"Select RouteID, RouteDescription, Supervisor, RouteType, Warehouse,   
   RouteArea, RowIsCurrent   
  , HistoricalHashKey, ChangeHashKey, UpdateHashKey   
  , HistoricalHashKeyASCII   
  , ChangeHashKeyASCII   
  , UpdateHashKeyASCII   
  , RowEndDate  
  , RowStartDate    
  FROM [dbo].[StgDimRoute];" ;  
    string UpdateSchemaName = "Updt";     
    string UpdateTableName = "UpdtRoute";  
    string UpdateConnection = "AWBIML";     
 string UpdateSQLStatement = @" UPDATE A    
  SET A.RouteArea = UA.RouteArea,    
    A.UpdateHashKey = UA.UpdateHashKey,    
    A.UpdtDtTm = CURRENT_TIMESTAMP    
  FROM Updt.UpdtRoute AS UA    
    JOIN Dbo.DimRoute AS A    
    ON UA.HistoricalHashKey = A.HistoricalHashKey    
  WHERE UpdateInPlace = 1;   
  UPDATE A    
  SET RowIsCurrent = 0,    
    A.RowEndDate = UA.RowEndDate,    
    A.UpdtDtTm = CURRENT_TIMESTAMP    
  FROM Updt.UpdtRoute UA    
    JOIN Dbo.DimRoute AS A    
    ON UA.HistoricalHashKey = A.HistoricalHashKey    
  WHERE A.RowIsCurrent = 1    
    AND UA.UpdateNewRow = 1;    
  INSERT INTO Dbo.DimRoute    
     (RouteID,    
     RouteDescription,    
     Supervisor,    
     RouteType,    
     Warehouse,    
     RouteArea,    
     RowIsCurrent,    
     RowStartDate,    
     RowEndDate,    
     HistoricalHashKey,    
     ChangeHashKey,    
     UpdateHashKey)    
  SELECT RouteID,    
     RouteDescription,    
     Supervisor,    
     RouteType,    
     Warehouse,    
     RouteArea,    
     1,    
     RowStartDate,    
     null,    
     HistoricalHashKey,    
     ChangeHashKey,    
     UpdateHashKey  
  FROM Updt.UpdtRoute WHERE UpdateNewRow = 1;" ;     
 #>  
 -->  
   <Packages>  
     <#=CallBimlScript("Dim2Hybrid.biml", PackageName, DstSchemaName, DstTableName, DstConnection, DataFlowSourceName, SrcConnection, SourceQuery, UpdateSchemaName, UpdateTableName, UpdateConnection, UpdateSQLStatement)#>  
   </Packages>  
 </Biml>  

Once I have my source view, dimension table, and update table in the database, the 3 Biml files added to my project, and BIDSHelper installed, all I have to do is right click on the CreateDim2Hybrid.Biml file and choose Generate SSIS packages to create my package.