与GROUP BY一起使用时COUNT（DISTINCT ..）的范围

Question

I'm doing something like follows (Example, getting distinct people named "Mark" by State):

Select count(distinct FirstName) FROM table
GROUP BY State

I think the group by query organization is done first, such that the distinct is only relative to each "group by"? Basically, can "Mark" show up as a "distinct" count in each group? This would "scope" my distinct expression to the group by rows only, I believe...

Answer 1

This may actually depend on where DISTINCT is used. For example, SELECT DISTINCT COUNT( would be different than SELECT COUNT(DISTINCT .

In this case, it will work as you want and get a count of distinct names in each group (even if the names are not distinct across groups).

Answer 2

Your understanding is correct. Group by says, essentially, to take a group of rows and aggregate them into one row (based on the criteria). All aggregation functions -- including count(distinct) -- summarize values in this group.

As a note, you are using the word "scope". Just so you know, this has a particular meaning in SQL. The meaning refers to the portions of the query where a column or table alias are understood by the compiler.