When you use a custom query to read data in a relational data object, the Data Integration Service can optimize the query by pushing the query to run in the database. The Data Integration Service can push the custom query if the query forms a valid subquery for the database. If the SQL syntax for the custom query is not valid in a subquery for the database, the resulting query fails to run.

If you push a custom query to a relational database other than IBM DB2, you must specify an alias for each expression in the select list that is not a column reference. The aliases allow the Data Integration Service to refer to the expressions in the select list.

See the database documentation for information about valid SQL syntax for aliases and subqueries.