对于rownum来说它是oracle系统顺序分配为从查询返回的行的编号，返回的第一行分配的是1，第二行是2，依此类推，这个伪字段可以用于限制查询返回的总行数，而且rownum不能以任何表的名称作为前缀。-Rownum oracle for the system in order for the distribution of which is from the number of rows returned by the query and returns the first line of distribution is 1, the second line is 2, and so on, the pseudo-field can be used to limit the query returns the total the number of rows, and rownum not in any table name as prefix.