Merge Transformation vs Union All Transformation in SSIS

The main difference is: Merge Transformation takes two inputs and gives one output whereas Union All Transformation takes multiple inputs and gives one output

Merge Transformation:

The Merge transformation combines two sorted datasets into a single dataset. The rows from each dataset are inserted into the output based on values in their key columns.

By including the Merge transformation in a data flow, you can perform the following tasks:

• Merge data from two data sources, such as tables and files.
• Create complex datasets by nesting Merge transformations.
• Remerge rows after correcting errors in the data.

Union All Transformation:

The Union All transformation combines multiple inputs into one output. For example, the outputs from five different Flat File sources can be inputs to the Union All transformation and combined into one output.

The Merge transformation is similar to the Union All transformations. Use the Union All transformation instead of the Merge transformation in the following situations:

• The transformation inputs are not sorted.
• The combined output does not need to be sorted.
• The transformation has more than two inputs.