sql语句如下：select * from (select a.out_id,a.cre_date credate,a.file_code filecode,a.title,a.need_grade,a.out_type type,a.subflag,a.out_code code,a.year year,a.out_code_seriation codeSeriation,a.flow_type flowtype from archives_out a where 1=1 and a.subflag='1' and a.out_type!='TN' union select a.in_id,a.cre_user_data credate,a.in_code filecode,a.title,a.need_grade,a.in_type type,a.subflag,a.title code,a.flow_type year,'22' codeSeriation,a.flow_type flowtype from archives_in a where 1=1 and a.subflag='1' union select a.sign_id,a.cre_date credate,a.flow_type filecode,a.title,a.need_grade,a.sign_type type,a.subflag,a.title code,a.flow_type year,'33' codeSeriation,a.flow_type flowtype from archives_sign a where 1=1 and a.subflag='1')where 1=1 ORDER BY credate desc执行后有如下异常，java.sql.SQLException: ORA-01427: single-row subquery returns more than one row请教大虾能够校正，谢谢。
where 1=1 and rownum=1 ORDER BY credate desc