[thelist] SQL query to create aging buckets
Paul Cowan
evolt at funkwit.com
Wed Dec 17 16:32:28 CST 2003
> How does 'Count(*)' know what it's counting? What is the wildcard
> matching? BOL isn't offering an answer
The number of rows grouped, basically.
If you do:
SELECT
firstname, count(*)
FROM
mytable
GROUP BY
firstname
Then you're getting the number of rows in mytable for each firstname,
basically.
Donald 5
Mickey 3
etc.
This is subtly different from naming a specific column, which is the
way normal aggregate functions (SUM, MIN, etc) work:
SELECT
firstname, count(petname)
FROM
mytable
GROUP BY
firstname
Here, the COUNT will only return the number of rows for each firstname
where petname is not null -- so if, on some rows, Donald doesn't have
any pets, you might only get 3 as the result for him.
COUNT(*) is the only aggregate that has this behaviour, because saying
SUM(*) doesn't really make much sense.
Paul.
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