COUNT() Function

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COUNT() Function

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ralm Hall of Fame Points: 3156 More actions · Answer 1

Hugo Kornelis (4/16/2013)
Good question, mediocre explanation - no errors, but complicating things more than needed.
There are three variations of COUNT:
* COUNT(*) - returns the number of rows, regardless of content.
* COUNT(expression) - evaluates expression for each row, and counts returns the number of non-NULL results. Both COUNT(columnname) and COUNT(1) are commonly used versions of this variation. The last one is too - it uses a complex expression that always returns NULL; COUNT returns the number of non-NULL results, which is 0.
* COUNT(DISTINCT expression) - as the previous one, but only the number of distinct values is returned. So if the same value is returned multiple times, it will be counted as 1. Rarely used in practise. The version in this question that used a constant expression is legal syntax, but will never be used in practise, as it will by definition return 1, except if the source of the query has no rows.

Thanks for the explanation Hugo.

I have a small doubt on

Both COUNT(columnname) and COUNT(1) are commonly used versions of this variation.

COUNT((columnname) and COUNT(1) returns the number of non-NULL results ??

create table #temp (SomeText varchar(20))

insert into #temp

select 'SomeText1'

union all

select NULL

union all

select 'SomeText2'

union all

select 'SomeText3'

union all

select 'SomeText3'

select COUNT(*) from #temp

select COUNT(1) from #temp

select COUNT(SomeText) from #temp

drop table #temp

The above code returns 5, 5, 4. Based on the output it looks like COUNT(*) and COUNT(1) returns the number of rows, regardless of content.

^{[font="Verdana"]Regards,
Rals[/font].}

Dana Medley SSCertifiable Points: 6764 More actions · Answer 2

Hugo Kornelis (4/16/2013)
Good question, mediocre explanation - no errors, but complicating things more than needed.
There are three variations of COUNT:
* COUNT(*) - returns the number of rows, regardless of content.
* COUNT(expression) - evaluates expression for each row, and counts returns the number of non-NULL results. Both COUNT(columnname) and COUNT(1) are commonly used versions of this variation. The last one is too - it uses a complex expression that always returns NULL; COUNT returns the number of non-NULL results, which is 0.
* COUNT(DISTINCT expression) - as the previous one, but only the number of distinct values is returned. So if the same value is returned multiple times, it will be counted as 1. Rarely used in practise. The version in this question that used a constant expression is legal syntax, but will never be used in practise, as it will by definition return 1, except if the source of the query has no rows.

Great question and great explanation Hugo. We can always COUNT() on you. :hehe:

Everything is awesome!

Sean Lange SSC Guru Points: 286590 More actions · Answer 3

kapil_kk (4/15/2013)
For the last query:
select COUNT(convert(int,NULL)) from #temp
It will return 0 because when you execute the query:
SELECT CONVERT(int,NULL) it will return NULL and as the return type of COUNT is INT so NULL will implicitly
converted to '0'.
Hope it will clear to you now Yogi 🙂

This isn't quite right Kapil. It is not an implicit conversion. What happens is you count NULL which will be zero. It doesn't actually matter what datatype you use, it just needs to have a datatype. It could just as easily be varchar or uniqueidentifier. The point is that you are casting NULL as a datatype and counting the number of NULL which we know will be ignored in a COUNT so the result is 0.

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Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 4

rals (4/16/2013)
(...)
The above code returns 5, 5, 4. Based on the output it looks like COUNT(*) and COUNT(1) returns the number of rows, regardless of content.

COUNT(*) returns the number of rows (after evaluating WHERE and GROUP BY if they are in the query) - by definition.

COUNT(1) is a specific use of the generic form COUNT(expression). The expression here is a very simple one - the constant 1. The definition of COUNT(expression) is to evaluate expression for each row - that will result in the value 1 for each row - and then count the number of non-NULL results. Since 1 is never NULL, the result of COUNT(1) will indeed always be equal to COUNT(*), allthough the "official" way to get there is a lot more complicated.

(And before you ask - the optimizer is smart enough to know that COUNT(1), just as COUNT(0) or COUNT('T'), or even COUNT(non-nullable column), are all equal to COUNT(*) and will use the same code path for all these versions).

And for the sake of completeness - the use of COUNT(1) instead of COUNT(*) to count the number of rows is very common. Long ago (I think it was in the days of SQL Server 6.0, maybe even before that), the parser used to first expand the * in COUNT(*) to a full column list, which would then be still ignored. Using COUNT(1) was more efficient because it avoided the hit on the metadata tables to get that columnlist. That has long since changed - but the myth lives on, and many people still use COUNT(1), claiming it to be better then COUNT(*).

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
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ralm Hall of Fame Points: 3156 More actions · Answer 5

Great explanation Hugo. Thanks.

Even I always use COUNT(1) though few times I heard both COUNT(*) and COUNT(1) are same. But today I more clear and thanks for you.

🙂

^{[font="Verdana"]Regards,
Rals[/font].}

Revenant SSC-Forever Points: 42467 More actions · Answer 6

Revenant

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Points: 42467

April 16, 2013 at 10:05 am

#1606896

Thanks for a nice one, Yogesh!

SQLRNNR SSC Guru Points: 281334 More actions · Answer 7

Thanks for the question

Jason...AKA CirqueDeSQLeil
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Rose Bud Hall of Fame Points: 3147 More actions · Answer 8

I thought the original poster's explanation was great: I found it very simple to understand and I liked the way each count statement was addressed individually.

TomThomson SSC Guru Points: 104773 More actions · Answer 9

Rose Bud (4/16/2013)
I thought the original poster's explanation was great: I found it very simple to understand and I liked the way each count statement was addressed individually.

The problem with the original explanation is this:

COUNT(convert(int,NULL)) - It will give count as 0 always. Don't know why it is giving 0 count however COUNT() function counts NULL values also.

The problem is that convert(int,NULL) is not a row but a column value, and the count function - although it counts rows all of whose values are null - doesn't count null values when it's counting the number of non-null occurences of a column. There is a big difference between count(*) )or count(<non-null constant>) from <something> and count(X) from <something> - the first is counting rows and doesn't care about nulls - a row is a row even if it contains nothing but nulls, the second doesn't see any nulls anyway, while the third (which is what was in question here, in this particular case) is counting non-null values.

Tom

chgn01 SSCarpal Tunnel Points: 4358 More actions · Answer 10

Thanks for the great question!

If you want retunr 5? here you go,

select COUNT(@@ERROR) FROM #temp

select COUNT(0) from #temp

select COUNT(100000000000000000000000000000) from #temp

select COUNT('ZZZZZZZZZZZZZZZZZZZZZZZZZZZZ') from #temp

select COUNT(-5.0) from #temp

select COUNT(+5.0) from #temp

select COUNT('') from #temp

select COUNT($10.0) from #temp

select COUNT(\.00) from #temp

--------------------------------------
;-)“Everything has beauty, but not everyone sees it.” ― Confucius

Venkat Jonnalagadda Mr or Mrs. 500 Points: 510 More actions · Answer 11

Hugo Kornelis (4/16/2013)
And for the sake of completeness - the use of COUNT(1) instead of COUNT(*) to count the number of rows is very common. Long ago (I think it was in the days of SQL Server 6.0, maybe even before that), the parser used to first expand the * in COUNT(*) to a full column list, which would then be still ignored. Using COUNT(1) was more efficient because it avoided the hit on the metadata tables to get that columnlist. That has long since changed - but the myth lives on, and many people still use COUNT(1), claiming it to be better then COUNT(*).

+1

You always seem to turn even the simplest of these Qotds to something so insightful that it forces me to read the discussions on every qotd...

You are awesome..!!!

___________________________________________________________________
If I can answer a question then anyone can answer it..trying to reverse the logic.. :hehe:

Miles Neale SSChampion Points: 13147 More actions · Answer 12

Mike Hays (4/16/2013)
Interesting, learned something, thanks for the good question....

Same Here!

Not all gray hairs are Dinosaurs!

kapil_kk SSC-Insane Points: 21316 More actions · Answer 13

Sean Lange (4/16/2013)
kapil_kk (4/15/2013)
For the last query:
select COUNT(convert(int,NULL)) from #temp
It will return 0 because when you execute the query:
SELECT CONVERT(int,NULL) it will return NULL and as the return type of COUNT is INT so NULL will implicitly
converted to '0'.
Hope it will clear to you now Yogi 🙂
This isn't quite right Kapil. It is not an implicit conversion. What happens is you count NULL which will be zero. It doesn't actually matter what datatype you use, it just needs to have a datatype. It could just as easily be varchar or uniqueidentifier. The point is that you are casting NULL as a datatype and counting the number of NULL which we know will be ignored in a COUNT so the result is 0.

Thanks for correcting me Sean

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kapil_kk SSC-Insane Points: 21316 More actions · Answer 14

Hugo Kornelis (4/16/2013)
rals (4/16/2013)
(...)
The above code returns 5, 5, 4. Based on the output it looks like COUNT(*) and COUNT(1) returns the number of rows, regardless of content.
COUNT(*) returns the number of rows (after evaluating WHERE and GROUP BY if they are in the query) - by definition.
COUNT(1) is a specific use of the generic form COUNT(expression). The expression here is a very simple one - the constant 1. The definition of COUNT(expression) is to evaluate expression for each row - that will result in the value 1 for each row - and then count the number of non-NULL results. Since 1 is never NULL, the result of COUNT(1) will indeed always be equal to COUNT(*), allthough the "official" way to get there is a lot more complicated.
(And before you ask - the optimizer is smart enough to know that COUNT(1), just as COUNT(0) or COUNT('T'), or even COUNT(non-nullable column), are all equal to COUNT(*) and will use the same code path for all these versions).
And for the sake of completeness - the use of COUNT(1) instead of COUNT(*) to count the number of rows is very common. Long ago (I think it was in the days of SQL Server 6.0, maybe even before that), the parser used to first expand the * in COUNT(*) to a full column list, which would then be still ignored. Using COUNT(1) was more efficient because it avoided the hit on the metadata tables to get that columnlist. That has long since changed - but the myth lives on, and many people still use COUNT(1), claiming it to be better then COUNT(*).

I was not aware of this things..

Thanks a lot Hugo for telling new things 🙂

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Thomas Abraham SSChampion Points: 10761 More actions · Answer 15

Thanks for the question. In general, I don't favor questions the require 57 answers to be evaluated. However, this one was OK as the code involved wasn't too complex.

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