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从SQL查询(Oracle)中字段的最小值获取所有行数据

min subquery group-by oracle sql

aCarella · 技术社区 · 9 年前

通过指定字段的最小值,我很难从SQL查询中获得一行所有信息。下面是我正在使用的数据和我试图获得的数据的基本示例:

SELECT 1 AS NUM_, 'ABC' AS LET_ FROM DUAL
UNION
SELECT 2 AS NUM_, 'DEF' AS LET_ FROM DUAL
UNION
SELECT 3 AS NUM_, 'GHI' AS LET_ FROM DUAL;

上述查询将产生以下结果:

NUM_ | LET_
-----------
  1  | ABC
  2  | DEF
  3  | GHI

我只想要包含数据的行 1 和 ABC .这是我尝试的,如下:

SELECT MIN(LN.NUM_) AS MIN_NUM, 
       LN.LET_ 
FROM   (SELECT 1 AS NUM_, 'ABC' AS LET_ FROM DUAL
        UNION
        SELECT 2 AS NUM_, 'DEF' AS LET_ FROM DUAL
        UNION
        SELECT 3 AS NUM_, 'GHI' AS LET_ FROM DUAL) LN
GROUP BY LET_;

但是上面仍然给出了所有的行。我如何只在中具有最小值的行上进行归零 NUM_ 专栏,给我 1. 和 基础知识 ?

3 回复 | 直到 9 年前

Pà®°à®¤à¯à®ªà¯ 9 年前

您的查询找到最小值 NUM_ 每人 LET_ .得到最小值 数量_ 在所有记录中,使用 ORDER BY & ROWNUM

SELECT LN.NUM_ AS MIN_NUM, 
       LN.LET_ 
FROM   (SELECT 1 AS NUM_, 'ABC' AS LET_ FROM DUAL
        UNION
        SELECT 2 AS NUM_, 'DEF' AS LET_ FROM DUAL
        UNION
        SELECT 3 AS NUM_, 'GHI' AS LET_ FROM DUAL
        ORDER BY LN.NUM_) LN
Where ROWNUM = 1

Gordon Linoff 9 年前

在Oracle 12C中,可以使用ANSI标准 fetch first row only 语法:

SELECT 1 AS NUM_, 'ABC' AS LET_ FROM DUAL
UNION ALL
SELECT 2 AS NUM_, 'DEF' AS LET_ FROM DUAL
UNION ALL
SELECT 3 AS NUM_, 'GHI' AS LET_ FROM DUAL
ORDER BY 1
FETCH FIRST 1 ROW ONLY;

Matthew McPeak 9 年前

正如已经指出的,有不同的方法:

一些方法:

FETCH FIRST 1 ROW ONLY
ORDER BY 鉴于 ROWNUM = 1
(我的补充) MAX KEEP
(我的添加)不相关子查询

仅获取第一行 是最简单的语法,并且性能足够好。但是如果您有一个大表,其他方法也值得考虑。

您的数据集太小,无法演示不同的性能结果,因此为了演示它们,让我们创建一个表 DBA_OBJECTS .

create table matt_test as SELECT * FROM dba_objects;
create unique index matt_test_u1 on matt_test (object_id);
exec dbms_stats.gather_table_stats(USER, 'MATT_TEST');

然后,尝试一些不同的方法,并使用DBMS_XPLAN进行测量:

最大保持

SELECT MAX (object_id) KEEP (DENSE_RANK FIRST ORDER BY object_id) object_id,
       MAX (object_name) KEEP (DENSE_RANK FIRST ORDER BY object_id) object_name
FROM   matt_test o;

------------------------------------------------------------------------------------------
| Id  | Operation          | Name      | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |      1 |        |      1 |00:00:00.29 |    8522 |
|   1 |  SORT AGGREGATE    |           |      1 |      1 |      1 |00:00:00.29 |    8522 |
|   2 |   TABLE ACCESS FULL| MATT_TEST |      1 |    555K|    555K|00:00:00.12 |    8522 |
------------------------------------------------------------------------------------------

先获取(注意内存使用情况)

SELECT object_id, object_name
FROM   matt_test o
order by object_id
fetch first 1 rows only;

---------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                | Name      | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |           |      1 |        |      1 |00:00:00.33 |    8522 |       |       |          |
|*  1 |  VIEW                    |           |      1 |      1 |      1 |00:00:00.33 |    8522 |       |       |          |
|*  2 |   WINDOW SORT PUSHED RANK|           |      1 |    555K|      1 |00:00:00.33 |    8522 |  2048 |  2048 | 2048  (0)|
|   3 |    TABLE ACCESS FULL     | MATT_TEST |      1 |    555K|    555K|00:00:00.12 |    8522 |       |       |          |
---------------------------------------------------------------------------------------------------------------------------

不相关子查询

select object_id, object_name
from matt_test
where object_id = ( SELECT min(object_id) FROM matt_test );

-------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |      1 |        |      1 |00:00:00.01 |       7 |
|   1 |  TABLE ACCESS BY INDEX ROWID | MATT_TEST    |      1 |      1 |      1 |00:00:00.01 |       7 |
|*  2 |   INDEX UNIQUE SCAN          | MATT_TEST_U1 |      1 |      1 |      1 |00:00:00.01 |       6 |
|   3 |    SORT AGGREGATE            |              |      1 |      1 |      1 |00:00:00.01 |       3 |
|   4 |     INDEX FULL SCAN (MIN/MAX)| MATT_TEST_U1 |      1 |      1 |      1 |00:00:00.01 |       3 |
-------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=)

因此,您可以看到一个不相关的子查询可以 最大保持 方法的性能比 FETCH FIRST ROWS

如果我写这篇文章时不考虑性能(例如,较小的数据集),默认方法是 取第一行 .