Magento 2 distinct SQL Query is used to return results as unique records for the given field by skipping duplicate values from the SQL Response.
If a Specific Column in a table contains, multiple duplicate values and you want to skip it and fetch only single records for the same column, You can use distinct keywords in a query to filter out duplicate values.
You can Learn about, A List of Direct SQL Queries in Magento 2.
Just use distinct keywords on a query like, $connection->select()->distinct()