一个（不错？）选项是使用两个选择作为CTE，然后您有各种选项可用，例如：

SQL> with
  2  c1 as (select count(*) cnt from emp),
  3  c2 as (select count(*) cnt from dept)
  4  select c1.cnt,
  5         c2.cnt,
  6         c1.cnt - c2.cnt as difference
  7  from c1 cross join c2;

       CNT        CNT DIFFERENCE
---------- ---------- ----------
        14          4         10

SQL>

如果您定期收集表的统计信息（并且在此期间表没有被更改（插入或删除）），您甚至可以使用user_tables视图的num_rows列：

SQL> select e.num_rows emp_rows,
  2         d.num_rows dept_rows,
  3         e.num_rows - d.num_rows difference
  4  from user_tables e cross join user_tables d
  5  where e.table_name = 'EMP'
  6    and d.table_name = 'DEPT';

  EMP_ROWS  DEPT_ROWS DIFFERENCE
---------- ---------- ----------

啊？没有数据？为什么？缺少统计数据！让我们收集它们：

SQL> exec dbms_stats.gather_table_stats('SCOTT', 'EMP');

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats('SCOTT', 'DEPT');

PL/SQL procedure successfully completed.

SQL> select e.num_rows emp_rows,
  2         d.num_rows dept_rows,
  3         e.num_rows - d.num_rows difference
  4  from user_tables e cross join user_tables d
  5  where e.table_name = 'EMP'
  6    and d.table_name = 'DEPT';

  EMP_ROWS  DEPT_ROWS DIFFERENCE
---------- ---------- ----------
        14          4         10

SQL>

现在它 * 工作 *。