Exists transformation in mapping data flow

APPLIES TO: Azure Data Factory Azure Synapse Analytics

Data flows are available both in Azure Data Factory and Azure Synapse Pipelines. This article applies to mapping data flows. If you are new to transformations, please refer to the introductory article Transform data using a mapping data flow.

The exists transformation is a row filtering transformation that checks whether your data exists in another source or stream. The output stream includes all rows in the left stream that either exist or don't exist in the right stream. The exists transformation is similar to SQL WHERE EXISTS and SQL WHERE NOT EXISTS.

Configuration

  1. Choose which data stream you're checking for existence in the Right stream dropdown.
  2. Specify whether you're looking for the data to exist or not exist in the Exist type setting.
  3. Select whether or not your want a Custom expression.
  4. Choose which key columns you want to compare as your exists conditions. By default, data flow looks for equality between one column in each stream. To compare via a computed value, hover over the column dropdown and select Computed column.

Exists settings

Multiple exists conditions

To compare multiple columns from each stream, add a new exists condition by clicking the plus icon next to an existing row. Each additional condition is joined by an "and" statement. Comparing two columns is the same as the following expression:

source1@column1 == source2@column1 && source1@column2 == source2@column2

Custom expression

To create a free-form expression that contains operators other than "and" and "equals to", select the Custom expression field. Enter a custom expression via the data flow expression builder by clicking on the blue box.

Exists custom settings

Broadcast optimization

Broadcast Join

In joins, lookups and exists transformation, if one or both data streams fit into worker node memory, you can optimize performance by enabling Broadcasting. By default, the spark engine will automatically decide whether or not to broadcast one side. To manually choose which side to broadcast, select Fixed.

It's not recommended to disable broadcasting via the Off option unless your joins are running into timeout errors.

Data flow script

Syntax

<leftStream>, <rightStream>
    exists(
        <conditionalExpression>,
        negate: { true | false },
        broadcast: { 'auto' | 'left' | 'right' | 'both' | 'off' }
    ) ~> <existsTransformationName>

Example

The below example is an exists transformation named checkForChanges that takes left stream NameNorm2 and right stream TypeConversions. The exists condition is the expression NameNorm2@EmpID == TypeConversions@EmpID && NameNorm2@Region == DimEmployees@Region that returns true if both the EMPID and Region columns in each stream matches. As we're checking for existence, negate is false. We aren't enabling any broadcasting in the optimize tab so broadcast has value 'none'.

In the UI experience, this transformation looks like the below image:

Exists example

The data flow script for this transformation is in the snippet below:

NameNorm2, TypeConversions
    exists(
        NameNorm2@EmpID == TypeConversions@EmpID && NameNorm2@Region == DimEmployees@Region,
	    negate:false,
	    broadcast: 'auto'
    ) ~> checkForChanges

Next steps

Similar transformations are Lookup and Join.