# Example: Creating a Conditional Constraint Using the Constraints Tab

In Merging the Contents of Repeating Elements, both of the input documents (PriceQuote.xml and AvailQuote.xml) share the common element widgetId. A constraint (as a condition) can be added to the join that specifies if the widgetId of the availRequest element is equal to the widgetId of the priceRequest element the merged repeating element quoteResponse be returned. Adding this constraint would change the resulting data as shown in Figure: Merging the Contents of Repeating Elements to the data shown in Figure: Using a Conditional Constraint to Merge Data.

Figure : Using a Conditional Constraint to Merge Data

For a step-by-step walk through of using the mapping functionality to create a join with a constraint, see Tutorial: Building Your First Data Transformation. Specifically, the join is created in Step 4: Mapping a Repeating Element (Join) in the Tutorial: Building Your First Data Transformation.