**
一、效率高的写法
** 1.无ORDER BY排序的写法。(效率最高) (经过测试，此方法成本最低，只嵌套一层，速度最快！即使查询的数据量再大，也几乎不受影响，速度依然！)
SELECT *
FROM (SELECT ROWNUM AS rowno, t.*
FROM emp t
WHERE hire_date BETWEEN TO_DATE ('20060501', 'yyyymmdd')
AND TO_DATE ('20060731', 'yyyymmdd')
AND ROWNUM <= 20) table_alias
WHERE table_alias.rowno >= 10;
2.有ORDER BY排序的写法。(效率较高) (经过测试，此方法随着查询范围的扩大，速度也会越来越慢哦！)
SELECT *
FROM (SELECT tt.*, ROWNUM AS rowno
FROM ( SELECT t.*
FROM emp t
WHERE hire_date BETWEEN TO_DATE ('20060501', 'yyyymmdd')
AND TO_DATE ('20060731', 'yyyymmdd')
ORDER BY create_time DESC, emp_no) tt
WHERE ROWNUM <= 20) table_alias
WHERE table_alias.rowno >= 10;1234567891011121314151617
**
