You can select unique values from sources in a customized data object or a relational data object instance with the select distinct option. When you enable select distinct, the Data Integration Service adds a SELECT DISTINCT statement to the default SQL query.
Use the select distinct option to filter source data. For example, you might use the select distinct option to extract unique customer IDs from a table that lists total sales. When you use the relational data object in a mapping, the Data Integration Service filters data earlier in the data flow, which can increase performance.