Comparison key is used to determine which rows correspond to each other. You can choose suitable constraints for comparison or specify custom comparison keys. Studio for SQL Server can select comparison key automatically if tables have matching primary key or unique constraint.
To choose a constraint pair, click the Drop-down button in the Comparison Key column on the Mapping page of the Data Comparison wizard and select the constraint pair you need.
To set a custom comparison key, click the Drop-down button in the Comparison Key column on the Mapping page of the Data Comparison wizard and select Custom. Then in the appeared Column Mapping dialog box select check boxes in the Key column to add columns to the comparison key.
Note that you can set custom comparison key or choose the needed constraint pair directly in the Comparison Key drop-down list of the Column Mapping dialog box.
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