“分组”自动保证“order by”? [英] Does "group by" automatically guarantee "order by"?
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问题描述
group by子句是否自动保证结果将按该键排序?换句话说,它是否足够写:
Does "group by" clause automatically guarantee that the results will be ordered by that key? In other words, is it enough to write:
select *
from table
group by a, b, c
或者必须写
select *
from table
group by a, b, c
order by a, b, c
我知道eg在MySQL我不必,但我想知道如果我可以依靠它跨越SQL实现。是否保证?
I know e.g. in MySQL I don't have to, but I would like to know if I can rely on it accross the SQL implementations. Is it guaranteed?
推荐答案
group by
。 DB被设计为尽可能快地获取数据,并且仅在必要时进行排序。
group by
does not order the data neccessarily. A DB is designed to grab the data as fast as possible and only sort if necessary.
如果您需要有保证的订单,请添加订单
。
So add the order by
if you need a guaranteed order.
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