Merge pull request #100362 from djpmsft/docUpdates

adding parameterized column name example

Author: v-albemi

articles/data-factory/media/data-flow/new-parameter-expression.png

@@ -1,18 +1,17 @@
 ---
-title: Azure Data Factory mapping data flow Parameters
+title: Parameterizing mapping data flows
 description: Learn how to parameterize a mapping data flow from data factory pipelines
 author: kromerm
 ms.author: makromer
+ms.reviewer: daperlov
 ms.service: data-factory
 ms.topic: conceptual
-ms.date: 06/10/2019
+ms.date: 01/07/2020
 ---
 
-# Mapping data flow Parameters
+# Parameterizing mapping data flows
 
-
-
-Mapping Data Flows in Azure Data Factory support the use of parameters. You can define parameters inside of your data flow definition, which you can then use throughout your expressions. The parameter values can be set by the calling pipeline via the Execute Data Flow activity. You have three options for setting the values in the data flow activity expressions:
+Mapping data flows in Azure Data Factory support the use of parameters. You can define parameters inside of your data flow definition, which you can then use throughout your expressions. The parameter values can be set by the calling pipeline via the Execute Data Flow activity. You have three options for setting the values in the data flow activity expressions:
 
 * Use the pipeline control flow expression language to set a dynamic value
 * Use the data flow expression language to set a dynamic value
@@ -23,25 +22,33 @@ Use this capability to make your data flows general-purpose, flexible, and reusa
 > [!NOTE]
 > To use pipeline control flow expressions, your data flow parameter must be of type string.
 
-## Create parameters in mapping data flow
+## Create parameters in a mapping data flow
 
-To add parameters to your data flow, click on the blank portion of the data flow canvas to see the general properties. In the settings pane, you will see a tab called 'Parameters'. Click the 'New' button to generate a new parameter. For each parameter, you must assign a name, select a type, and optionally set a default value.
+To add parameters to your data flow, click on the blank portion of the data flow canvas to see the general properties. In the settings pane, you will see a tab called **Parameter**. Select **New** to generate a new parameter. For each parameter, you must assign a name, select a type, and optionally set a default value.
 
 ![Create Data Flow parameters](media/data-flow/create-params.png "Create Data Flow parameters")
 
-Parameters can be utilized in any data flow expression. Parameters begin with $ and are immutable. You will find the list of available parameters inside of the Expression Builder under the 'Parameters' tab.
+## Use parameters in a mapping data flow 
+
+Parameters can be referenced in any data flow expression. Parameters begin with $ and are immutable. You will find the list of available parameters inside of the Expression Builder under the **Parameters** tab.
 
 ![Data flow parameter expression](media/data-flow/parameter-expression.png "Data flow parameter expression")
 
-## Use parameters in your data flow
+You can quickly add additional parameters by selecting **New parameter** and specifying the name and type.
+
+![Data flow parameter expression](media/data-flow/new-parameter-expression.png "Data flow parameter expression")
+
+### Passing in a column name as a parameter
+
+A common pattern is to pass in a column name as a parameter value. To reference the column associated with the parameter, use the `byName()` function. Remember to cast the column to its appropriate type with a casting function such as `toString()`.
 
-* You can use parameter values inside of your transformation expressions. You will find the parameter list under the Parameters tab in the Expression Builder. ![Use Data Flow parameters](media/data-flow/params9.png "Use Data Flow parameters")
+For example, if you wanted to map a string column based upon a parameter `columnName`, you can add a derived column transformation equal to `toString(byName($columnName))`.
 
-* Parameters are also used to configure dynamic values for your Source and Sink transformation settings. When you click inside of configurable fields, you will see the "Add dynamic contect" link appear. Clicking there will take you to an Expression Builder where you can use parameters to use dynamic values. ![Data flow dynamic content](media/data-flow/params6.png "Data flow dynamic content")
+![Passing in a column name as a parameter](media/data-flow/parameterize-column-name.png "Passing in a column name as a paramete")
 
-## Set mapping data flow parameters from pipeline
+## Assign parameter values from a pipeline
 
-Once you've created your data flow with parameters, you can execute it from a pipeline with the Execute Data Flow Activity. After you add the activity to your pipeline canvas, you will be presented with the available data flow parameters in the activity's 'Parameters' tab.
+Once you've created your data flow with parameters, you can execute it from a pipeline with the Execute Data Flow Activity. After you add the activity to your pipeline canvas, you will be presented with the available data flow parameters in the activity's **Parameters** tab.
 
 ![Setting a Data Flow parameter](media/data-flow/parameter-assign.png "Setting a Data Flow parameter")