group by 子句中的列顺序是否重要?
如果我有两列，一列具有非常高的基数，另一列具有非常低的基数(唯一的值数)，我按哪个顺序分组有关系吗?
If I have two columns, one with very high cardinality and one with very low cardinality (unique # of values), does it matter in which order I group by?
这是一个例子:
select
dimensionName,
dimensionCategory,
sum(someFact)
from SomeFact f
join SomeDim d on f.dimensionKey = d.dimensionKey
group by
d.dimensionName, -- large number of unique values
d.dimensionCategory -- small number of unique values
是否存在重要的情况?
推荐答案
不，顺序与 GROUP BY 子句无关.
No, the order doesn't matter for the GROUP BY clause.
MySQL 和 SQLite 是我所知道的唯一允许您选择从 group by(非标准，不可移植)中省略的列的数据库，但顺序也不重要.
MySQL and SQLite are the only databases I'm aware of that allow you to select columns which are omitted from the group by (non-standard, not portable) but the order doesn't matter there either.
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