Among other things, a method of generating a computer program based on an SQL query includes receiving a SQL query, including a reference to a first data set stored at a first data source, and including a reference to a second data set stored at a second data source different from the first data source, determining that the SQL query includes two or more commands, the commands including a first union-type operation, and a first aggregation operation, and determining that the SQL query describes that the first union-type operation shall be applied to at least a portion of data from the first data set, and applied to at least a portion of data from the second data set, determining that the SQL query describes that the first aggregation operation shall be applied to data resulting from the first union-type operation, and generating the computer program.