如何在订购后限制Oracle查询返回的行数？

Question

有没有办法让Oracle查询表现得像包含一个MySQL limit子句？

在MySQL,我可以这样做:

select * 
from sometable
order by name
limit 20,10

获得第21行到第30行(跳过前20行,给出下一行10).在行之后选择行order by,因此它实际上按字母顺序从第20个名称开始.

在Oracle,人们提到的唯一的事情是rownum伪列,但它之前 进行了评估order by,这意味着:

select * 
from sometable
where rownum <= 10
order by name

将返回按名称排序的十行的随机集合,这通常不是我想要的.它也不允许指定偏移量.

Answer 1

您可以使用子查询

select *
from  
( select * 
  from emp 
  order by sal desc ) 
where ROWNUM <= 5;

还可以查看主题On ROWNUM并在Oracle/AskTom上限制结果以获取更多信息.

更新:为了限制下限和上限的结果,事情会变得更加臃肿

select * from 
( select a.*, ROWNUM rnum from 
  ( <your_query_goes_here, with order by> ) a 
  where ROWNUM <= :MAX_ROW_TO_FETCH )
where rnum  >= :MIN_ROW_TO_FETCH;

(从指定的AskTom文章复制)

更新2:从Oracle 12c(12.1)开始,有一种语法可用于限制行或从偏移量开始.

SELECT * 
FROM   sometable
ORDER BY name
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

有关更多示例,请参阅此答案.感谢Krumia的暗示.

Answer 2

从Oracle 12C R1(12.1)开始,那里是一个行限制性条款.它不使用熟悉的LIMIT语法,但它可以通过更多选项更好地完成工作.您可以在此处找到完整语法.

要回答原始问题,这是查询:

SELECT * 
FROM   sometable
ORDER BY name
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

(对于早期的Oracle版本,请参阅此问题中的其他答案)

例子:

以下示例引用链接页面,以防止链接腐烂.

建立

CREATE TABLE rownum_order_test (
  val  NUMBER
);

INSERT ALL
  INTO rownum_order_test
SELECT level
FROM   dual
CONNECT BY level <= 10;

COMMIT;

桌子上有什么？

SELECT val
FROM   rownum_order_test
ORDER BY val;

       VAL
----------
         1
         1
         2
         2
         3
         3
         4
         4
         5
         5
         6
         6
         7
         7
         8
         8
         9
         9
        10
        10

20 rows selected.

获取第一N行

SELECT val
FROM   rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS ONLY;

       VAL
----------
        10
        10
         9
         9
         8

5 rows selected.

获得第一N行中,如果N^日行有关系,让所有的束缚行

SELECT val
FROM   rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS WITH TIES;

       VAL
----------
        10
        10
         9
         9
         8
         8

6 rows selected.

排名最高x的行数

SELECT val
FROM   rownum_order_test
ORDER BY val
FETCH FIRST 20 PERCENT ROWS ONLY;

       VAL
----------
         1
         1
         2
         2

4 rows selected.

使用偏移量,对分页非常有用

SELECT val
FROM   rownum_order_test
ORDER BY val
OFFSET 4 ROWS FETCH NEXT 4 ROWS ONLY;

       VAL
----------
         3
         3
         4
         4

4 rows selected.

您可以将偏移量与百分比组合

SELECT val
FROM   rownum_order_test
ORDER BY val
OFFSET 4 ROWS FETCH NEXT 20 PERCENT ROWS ONLY;

       VAL
----------
         3
         3
         4
         4

4 rows selected.

Answer 3

我对以下方法进行了一些性能测试:

Asktom

select * from (
  select a.*, ROWNUM rnum from (
    <select statement with order by clause>
  ) a where rownum <= MAX_ROW
) where rnum >= MIN_ROW

分析

select * from (
  <select statement with order by clause>
) where myrow between MIN_ROW and MAX_ROW

简短的选择

select * from (
  select statement, rownum as RN with order by clause
) where a.rn >= MIN_ROW and a.rn <= MAX_ROW

结果

表有1000万条记录,排序在未编入索引的日期时间行:

• 解释计划显示所有三个选择的相同值(323168)
• 但获胜者是AskTom(分析紧随其后)

选择前10行:

• AskTom:28-30秒
• 分析:33-37秒
• 短期替代方案:110-140秒

选择100,000到100,010之间的行:

• AskTom:60秒
• 分析:100秒

选择9,000,000到9,000,010之间的行:

• AskTom:130秒
• 分析:150秒

Answer 4

只有一个嵌套查询的分析解决方案:

SELECT * FROM
(
   SELECT t.*, Row_Number() OVER (ORDER BY name) MyRow FROM sometable t
) 
WHERE MyRow BETWEEN 10 AND 20;

Rank()Row_Number()如果名称有重复值,则可以替换但可能会返回比预期更多的记录.

Answer 5

在Oracle 12c上(参见SQL参考中的行限制子句):

SELECT * 
FROM sometable
ORDER BY name
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;

Answer 6

SQL标准

由于版本 12c Oracle 支持 SQL:2008 标准，它提供以下语法来限制 SQL 结果集：

SELECT
    title
FROM
    post
ORDER BY
    id DESC
FETCH FIRST 50 ROWS ONLY

Oracle 11g 及更早版本

在版本 12c 之前，要获取Top-N记录，您必须使用派生表和ROWNUM伪列：

SELECT *
FROM (
    SELECT
        title
    FROM
        post
    ORDER BY
        id DESC
)
WHERE ROWNUM <= 50

Answer 7

在Oracle中,带有排序的分页查询非常棘手.

Oracle提供了一个ROWNUM伪列,它返回一个数字,指示数据库从一个表或一组连接视图中选择行的顺序.

ROWNUM是一个伪列,让许多人陷入困境.ROWNUM值不会永久分配给一行(这是一种常见的误解).实际分配ROWNUM值时可能会造成混淆.在通过查询的筛选谓词但在查询聚合或排序之前,将 ROWNUM值分配给行.

更重要的是,ROWNUM值仅在分配后才会递增.

这就是followin查询不返回任何行的原因:

 select * 
 from (select *
       from some_table
       order by some_column)
 where ROWNUM <= 4 and ROWNUM > 1; 

查询结果的第一行不传递ROWNUM> 1谓词,因此ROWNUM不会增加到2.因此,没有ROWNUM值大于1,因此查询不返回任何行.

正确定义的查询应如下所示:

select *
from (select *, ROWNUM rnum
      from (select *
            from skijump_results
            order by points)
      where ROWNUM <= 4)
where rnum > 1; 

在我关于Vertabelo博客的文章中了解有关分页查询的更多信息:

• Oracle ROWNUM解释
• 前N个和分页查询

Answer 8

作为公认答案的扩展，Oracle 在内部使用ROW_NUMBER/RANK函数。OFFSET FETCH语法是一种语法糖。

可以通过使用DBMS_UTILITY.EXPAND_SQL_TEXT程序来观察：

准备样品：

CREATE TABLE rownum_order_test (
  val  NUMBER
);

INSERT ALL
  INTO rownum_order_test
SELECT level
FROM   dual
CONNECT BY level <= 10;
COMMIT;

询问：

SELECT val
FROM   rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS ONLY;

是常规的：

SELECT "A1"."VAL" "VAL" 
FROM  (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
               ROW_NUMBER() OVER ( ORDER BY "A2"."VAL" DESC ) "rowlimit_$$_rownumber" 
      FROM "ROWNUM_ORDER_TEST" "A2") "A1" 
WHERE "A1"."rowlimit_$$_rownumber"<=5 ORDER BY "A1"."rowlimit_$_0" DESC;

db<>小提琴演示

获取扩展的 SQL 文本：

declare
  x VARCHAR2(1000);
begin
 dbms_utility.expand_sql_text(
        input_sql_text => '
          SELECT val
          FROM   rownum_order_test
          ORDER BY val DESC
          FETCH FIRST 5 ROWS ONLY',
        output_sql_text => x);

  dbms_output.put_line(x);
end;
/

WITH TIES扩展为RANK：

declare
  x VARCHAR2(1000);
begin
 dbms_utility.expand_sql_text(
        input_sql_text => '
          SELECT val
          FROM   rownum_order_test
          ORDER BY val DESC
          FETCH FIRST 5 ROWS WITH TIES',
        output_sql_text => x);

  dbms_output.put_line(x);
end;
/

SELECT "A1"."VAL" "VAL" 
FROM  (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
              RANK() OVER ( ORDER BY "A2"."VAL" DESC ) "rowlimit_$$_rank" 
       FROM "ROWNUM_ORDER_TEST" "A2") "A1" 
WHERE "A1"."rowlimit_$$_rank"<=5 ORDER BY "A1"."rowlimit_$_0" DESC

和偏移：

declare
  x VARCHAR2(1000);
begin
 dbms_utility.expand_sql_text(
        input_sql_text => '
          SELECT val
FROM   rownum_order_test
ORDER BY val
OFFSET 4 ROWS FETCH NEXT 4 ROWS ONLY',
        output_sql_text => x);

  dbms_output.put_line(x);
end;
/


SELECT "A1"."VAL" "VAL" 
FROM  (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
             ROW_NUMBER() OVER ( ORDER BY "A2"."VAL") "rowlimit_$$_rownumber" 
       FROM "ROWNUM_ORDER_TEST" "A2") "A1" 
       WHERE "A1"."rowlimit_$$_rownumber"<=CASE  WHEN (4>=0) THEN FLOOR(TO_NUMBER(4)) 
             ELSE 0 END +4 AND "A1"."rowlimit_$$_rownumber">4 
ORDER BY "A1"."rowlimit_$_0"

Answer 9

对于 21c 版本，您可以简单地应用限制，如下所示：

select * from course where ROWNUM <=10;

Answer 10

少SELECT语句.此外,性能消耗较少.致记:anibal@upf.br

SELECT *
    FROM   (SELECT t.*,
                   rownum AS rn
            FROM   shhospede t) a
    WHERE  a.rn >= in_first
    AND    a.rn <= in_first;