我希望你能帮我做作业:)
我的版本完美无缺.
例如前三名：
SELECT name,salary
FROM staff
WHERE salary IN ( SELECT *
FROM ( SELECT salary
FROM staff
ORDER BY salary DESC )
WHERE ROWNUM <= 3 )
ORDER BY salary DESC
;
请注意,这将输出位于前3名且薪水相同的员工.
1: Mike,4080
2: Steve,2800
2: Susan,2800
2: Jack,2800
3: Chloe,1400
但现在我们的老师不允许我们使用ROWNUM.
我搜索得很远,没有找到任何可用的东西.
贾斯汀洞穴的暗示,这是我的第二个解决方案.
首先我试过这个：
SELECT name,salary,( rank() OVER ( ORDER BY salary DESC ) ) as myorder FROM staff WHERE myorder <= 3 ;
错误消息是：“myorder：invalid identifier”
感谢DCookie,现在很清楚：
“[…] Analytics are applied AFTER
the where clause is evaluated,which
is why you get the error that myorder
is an invalid identifier.”
包装SELECT解决了这个问题：
SELECT * FROM ( SELECT name,rank() OVER ( ORDER BY salary DESC ) as myorder FROM staff ) WHERE myorder <= 3 ;
我的老师再次罢工,不允许这种奇特的分析功能.
来自@Justin Caves的第三个解决方案.
“If analytic functions are also
disallowed,the other option I Could
imagine– one that you would never,
ever,ever actually write in practice,
would be something like”
SELECT name,salary
FROM staff s1
WHERE (SELECT COUNT(*)
FROM staff s2
WHERE s1.salary < s2.salary) <= 3
因为这是作业,提示而不是答案.您将需要使用分析函数. ROW_NUMBER,RANK或DENSE_RANK可以根据您想要处理关系的方式工作.
如果也不允许分析函数,我可以想象的另一个选项 – 你永远不会,实际上在实践中写的那个,就像是
SELECT name,salary
FROM staff s1
WHERE (SELECT COUNT(*)
FROM staff s2
WHERE s1.salary < s2.salary) <= 3
关于性能,我不会依赖查询计划中的COST编号 – 这只是一个估计,通常不可能比较不同sql语句的计划之间的成本.你可以更好地查看查询实际执行的一致获取次数,并考虑查询性能随着表中行数的增加而缩放的情况.第三个选项的效率远远低于其他两个选项,因为它需要两次扫描STAFF表.
我没有你的STAFF表,所以我将使用SCott模式中的EMP表
分析函数解决方案实际上与ROWNUM解决方案一样进行7次一致性获取
Wrote file afiedt.buf 1 select ename,sal 2 from( select ename,3 sal,4 rank() over (order by sal) rnk 5 from emp ) 6* where rnk <= 3 sql> / ENAME SAL ---------- ---------- smith 800 SM0 950 AdamS 1110 Execution Plan ---------------------------------------------------------- Plan hash value: 3291446077 -------------------------------------------------------------------------------- - | Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time | -------------------------------------------------------------------------------- - | 0 | SELECT STATEMENT | | 14 | 672 | 4 (25)| 00:00:01 |* 1 | VIEW | | 14 | 672 | 4 (25)| 00:00:01 |* 2 | WINDOW SORT PUSHED RANK| | 14 | 140 | 4 (25)| 00:00:01 | 3 | TABLE ACCESS FULL | EMP | 14 | 140 | 3 (0)| 00:00:01 -------------------------------------------------------------------------------- - Predicate information (identified by operation id): --------------------------------------------------- 1 - filter("RNK"<=3) 2 - filter(RANK() OVER ( ORDER BY "SAL")<=3) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 7 consistent gets 0 physical reads 0 redo size 668 bytes sent via sql*Net to client 524 bytes received via sql*Net from client 2 sql*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 3 rows processed sql> select ename,sal 3 from emp 4 order by sal ) 5 where rownum <= 3; ENAME SAL ---------- ---------- smith 800 SM0 950 AdamS 1110 Execution Plan ---------------------------------------------------------- Plan hash value: 1744961472 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 105 | 4 (25)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | VIEW | | 14 | 490 | 4 (25)| 00:00:01 | |* 3 | SORT ORDER BY STOPKEY| | 14 | 140 | 4 (25)| 00:00:01 | | 4 | TABLE ACCESS FULL | EMP | 14 | 140 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------- Predicate information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<=3) 3 - filter(ROWNUM<=3) Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 7 consistent gets 0 physical reads 0 redo size 668 bytes sent via sql*Net to client 524 bytes received via sql*Net from client 2 sql*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 3 rows processed
但是,COUNT(*)解决方案实际上会执行99次一致性获取,并且必须对表进行两次完整扫描,因此效率低10倍以上.随着表中行数的增加,它的扩展性会更差
sql> select ename,sal 2 from emp e1 3 where (select count(*) from emp e2 where e1.sal < e2.sal) <= 3; ENAME SAL ---------- ---------- JOnes 2975 SCott 3000 KING 5000 FORD 3000 FOO Execution Plan ---------------------------------------------------------- Plan hash value: 2649664444 ---------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time | ---------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 140 | 24 (0)| 00:00:01 | |* 1 | FILTER | | | | | | | 2 | TABLE ACCESS FULL | EMP | 14 | 140 | 3 (0)| 00:00:01 | | 3 | SORT AGGREGATE | | 1 | 4 | | | |* 4 | TABLE ACCESS FULL| EMP | 1 | 4 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------- Predicate information (identified by operation id): --------------------------------------------------- 1 - filter( (SELECT COUNT(*) FROM "EMP" "E2" WHERE "E2"."SAL">:B1)<=3) 4 - filter("E2"."SAL">:B1) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 99 consistent gets 0 physical reads 0 redo size 691 bytes sent via sql*Net to client 524 bytes received via sql*Net from client 2 sql*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 5 rows processed
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