hop-user-manual/modules/ROOT/pages/plugins/transforms/mergerows.adoc

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= Merge rows (diff)

== Description

The Merge rows (diff) transform compares and merges data within two rows of data. This transform is useful for comparing data collected at two different times. For example, the source system of your data warehouse might not contain a timestamp of the last data update. You could use this transform to compare the two data streams and and merge the dates and timestamps in the rows.

Based on keys for comparison, this transform merges reference rows (previous data) with compare rows (new data) and creates merged output rows. A flag in the row indicates how the values were compared and merged. Flag values include:

* **identical**: The key was found in both rows, and the compared values are identical.

* **changed**: The key was found in both rows, but one or more compared values are different.

* **new**: The key was not found in the reference rows.

* **deleted**: The key was not found in the compare rows.

If the row's flag is **identical** or **deleted**, the merged output rows are based on the reference rows.

For **new** or **changed** rows, the merged output rows are based on the compare rows.

You can also send values from the merged and flagged rows to a subsequent transform in your pipeline, such as the Switch-Case transform or the Synchronize after merge transform. In the subsequent transform, you can use the flag field generated by **Merge rows (diff)** to control updates/inserts/deletes on a target table.

== Options

[width="90%", options="header"]
|===
|Option|Description
|Transform name|Name of the transform.
|Reference rows origin|Specify the transform origin for the reference rows <- Stream with original rows, or rows you want to compare the new rows to.
|Compare rows origin|Specify the transform origin for the compare rows.<- Stream with new rows
|Flag fieldname|Specify the name of the flag field on the output stream.
|Keys to match|Specify fields containing the keys on which to match;click Get key fields to insert all of the fields originating from the reference rows transform
|Values to compare|Specify fields contaning the values to compare; click Get value fields to insert all of the fields from the originating value rows transform.  Key fields do not need to be specified here.
|===

== Metadata Injection Support

All fields of this transform support metadata injection. You can use this transform with ETL Metadata Injection to pass metadata to your pipeline at runtime.