"group by" 是否自动保证 "order by"？

Question

"group by" 子句是否自动保证结果将按该键排序？换句话说，这样写就够了吗：

select * 
from table
group by a, b, c

还是必须写

select * 
from table
group by a, b, c
order by a, b, c

我知道，例如在 MySQL 中，我不必这样做，但我想知道我是否可以在 SQL 实现中依赖它。有保障吗？

Answer 1

group by不一定对数据进行排序。数据库旨在尽可能快地获取数据，并且仅在必要时才进行排序。

因此，如果您需要保证订单，请添加 order by。

Answer 2

绝对不是。我有过这样的经历，一旦我的一个查询突然开始 return 未排序的结果，随着 table 中的数据增长。

Answer 3

这取决于记录的数量。当记录较少时，Group by自动排序。当记录较多（超过15条）时需要添加Order by clause

Answer 4

我试过了。 Msdn 的 Adventureworks 数据库。

select HireDate, min(JobTitle)
from AdventureWorks2016CTP3.HumanResources.Employee
group by HireDate

结果：

2009-01-10生产技术员-WC40

2009-01-11应用专家

2009-01-12财务总监助理

2009-01-13生产技术员-WC50<

它returns 对 hiredate 的数据进行了排序，但在任何情况下都不依赖 GROUP BY 进行排序。

例如；索引可以更改此排序数据。

我添加了以下索引（hiredate, jobtitle）

CREATE NONCLUSTERED INDEX NonClusturedIndex_Jobtitle_hireddate ON [HumanResources].[Employee]
(
    [JobTitle] ASC,
    [HireDate] ASC
)

结果将随着相同的 select 查询而改变；

2006-06-30 生产技术员-WC60

2007-01-26 营销助理

2007-11-11 工程经理

2007-12-05 高级工具设计师

2007-12-11 工具设计师

2007-12-20 营销经理

2007-12-26 生产主管-WC60

您可以在以下地址下载Adventureworks2016

https://www.microsoft.com/en-us/download/details.aspx?id=49502

Answer 5

An efficient implementation of group by would perform the group-ing by sorting the data internally. That's why some RDBMS return sorted output when group-ing. Yet, the SQL specs don't mandate that behavior, so unless explicitly documented by the RDBMS vendor I wouldn't bet on it to work (tomorrow). OTOH, if the RDBMS implicitly does a sort it might also be smart enough to then optimize (away) the redundant order by. @jimmyb

一个使用 PostgreSQL 的例子证明了这个概念

创建一个 table 包含 100 万条记录，随机日期在从今天到 90 日的一天范围内，并按日期索引

CREATE TABLE WITHDRAW AS
  SELECT (random()*1000000)::integer AS IDT_WITHDRAW,
    md5(random()::text) AS NAM_PERSON,
    (NOW() - ( random() * (NOW() + '90 days' - NOW()) ))::timestamp AS DAT_CREATION, -- de hoje a 90 dias atras
    (random() * 1000)::decimal(12, 2) AS NUM_VALUE
  FROM generate_series(1,1000000);

CREATE INDEX WITHDRAW_DAT_CREATION ON WITHDRAW(DAT_CREATION);

按日期分组，按日期截断，select 按日期限制在两天范围内

EXPLAIN 
SELECT
    DATE_TRUNC('DAY', W.dat_creation), COUNT(1), SUM(W.NUM_VALUE)
FROM WITHDRAW W
WHERE W.dat_creation >= (NOW() - INTERVAL '2 DAY')::timestamp
AND W.dat_creation < (NOW() - INTERVAL '1 DAY')::timestamp
GROUP BY 1

HashAggregate  (cost=11428.33..11594.13 rows=11053 width=48)
  Group Key: date_trunc('DAY'::text, dat_creation)
  ->  Bitmap Heap Scan on withdraw w  (cost=237.73..11345.44 rows=11053 width=14)
        Recheck Cond: ((dat_creation >= ((now() - '2 days'::interval))::timestamp without time zone) AND (dat_creation < ((now() - '1 day'::interval))::timestamp without time zone))
        ->  Bitmap Index Scan on withdraw_dat_creation  (cost=0.00..234.97 rows=11053 width=0)
              Index Cond: ((dat_creation >= ((now() - '2 days'::interval))::timestamp without time zone) AND (dat_creation < ((now() - '1 day'::interval))::timestamp without time zone))

使用更大的限制日期范围，它选择应用 SORT

EXPLAIN 
SELECT
    DATE_TRUNC('DAY', W.dat_creation), COUNT(1), SUM(W.NUM_VALUE)
FROM WITHDRAW W
WHERE W.dat_creation >= (NOW() - INTERVAL '60 DAY')::timestamp
AND W.dat_creation < (NOW() - INTERVAL '1 DAY')::timestamp
GROUP BY 1

GroupAggregate  (cost=116522.65..132918.32 rows=655827 width=48)
  Group Key: (date_trunc('DAY'::text, dat_creation))
  ->  Sort  (cost=116522.65..118162.22 rows=655827 width=14)
        Sort Key: (date_trunc('DAY'::text, dat_creation))
        ->  Seq Scan on withdraw w  (cost=0.00..41949.57 rows=655827 width=14)
              Filter: ((dat_creation >= ((now() - '60 days'::interval))::timestamp without time zone) AND (dat_creation < ((now() - '1 day'::interval))::timestamp without time zone))

只要在最后加上ORDER BY 1即可（没有显着差异）

GroupAggregate  (cost=116522.44..132918.06 rows=655825 width=48)
  Group Key: (date_trunc('DAY'::text, dat_creation))
  ->  Sort  (cost=116522.44..118162.00 rows=655825 width=14)
        Sort Key: (date_trunc('DAY'::text, dat_creation))
        ->  Seq Scan on withdraw w  (cost=0.00..41949.56 rows=655825 width=14)
              Filter: ((dat_creation >= ((now() - '60 days'::interval))::timestamp without time zone) AND (dat_creation < ((now() - '1 day'::interval))::timestamp without time zone))

PostgreSQL 10.3

Answer 6

这取决于数据库供应商。

例如 PostgreSQL 不会自动对分组结果进行排序。这里必须使用order by来对数据进行排序。

但 Sybase 和 Microsoft SQL 服务器可以。这里可以使用order by来更改默认排序。

"group by" 是否自动保证 "order by"？

Does "group by" automatically guarantee "order by"?

sql

database

group-by

database-agnostic