count(*) vs count(column-name) - which is more correct?
COUNT(*)
counts all rowsCOUNT(column)
counts non-NULLs onlyCOUNT(1)
is the same asCOUNT(*)
because 1 is a non-null expressions
Your use of COUNT(*)
or COUNT(column)
should be based on the desired output only.
What is the difference between COUNT(*) and COUNT(table.ColumnName)?
The difference between these two is not (primarily) performance. They count different things:
COUNT(*)
counts the rows in your table.
COUNT(column)
counts the entries in a column - ignoring null
values.
Of course there will be performance differences between these two, but that is to be expected if they are doing different things. Especially when the column allows null
-values, the query will take longer than on a column that does not (or COUNT(*)
).
SQL Query: Which one should i use? count( columnname ) or count(1)
There can be differences between count(*)
and count(column)
. count(*)
is often fastest for reasons discussed here. Basically, with count(column)
the database has to check if column
is null or not in each row. With count(column)
it just returns the total number of rows in the table which is probably has on hand. The exact details may depend on the database and the version of the database.
Short answer: use count(*)
or count(1)
. Hell, forget the count and select userid
.
You should also make sure the where
clause is performing well and that its using an index. Look into EXPLAIN
.
What is the difference between count (*) and count(attribute_name)?
Imagine this table:
select Count(TelephoneNumber) from Calls -- returns 3
select Count(*) from Calls -- returns 4
count(column_name) also counts duplicate values. Consider:
select Count(TelephoneNumber) from Calls -- returns 4
In SQL, what's the difference between count(column) and count(*)?
count(*)
counts NULLs and count(column)
does not
[edit] added this code so that people can run it
create table #bla(id int,id2 int)
insert #bla values(null,null)
insert #bla values(1,null)
insert #bla values(null,1)
insert #bla values(1,null)
insert #bla values(null,1)
insert #bla values(1,null)
insert #bla values(null,null)
select count(*),count(id),count(id2)
from #bla
results
7 3 2
Count(*) vs Count(1) - SQL Server
There is no difference.
Reason:
Books on-line says "
COUNT ( { [ [ ALL | DISTINCT ] expression ] | * } )
"
"1" is a non-null expression: so it's the same as COUNT(*)
.
The optimizer recognizes it for what it is: trivial.
The same as EXISTS (SELECT * ...
or EXISTS (SELECT 1 ...
Example:
SELECT COUNT(1) FROM dbo.tab800krows
SELECT COUNT(1),FKID FROM dbo.tab800krows GROUP BY FKID
SELECT COUNT(*) FROM dbo.tab800krows
SELECT COUNT(*),FKID FROM dbo.tab800krows GROUP BY FKID
Same IO, same plan, the works
Edit, Aug 2011
Similar question on DBA.SE.
Edit, Dec 2011
COUNT(*)
is mentioned specifically in ANSI-92 (look for "Scalar expressions 125
")
Case:
a) If COUNT(*) is specified, then the result is the cardinality of T.
That is, the ANSI standard recognizes it as bleeding obvious what you mean. COUNT(1)
has been optimized out by RDBMS vendors because of this superstition. Otherwise it would be evaluated as per ANSI
b) Otherwise, let TX be the single-column table that is the
result of applying theto each row of T
and eliminating null values. If one or more null values are
eliminated, then a completion condition is raised: warning-
In SQL is there a difference between count(*) and count( fieldname )
Count(*) counts all records, including nulls, whereas Count(fieldname) does not include nulls.
count(*) and count(column_name), what's the diff?
COUNT(*)
counts all rows in the result set (or group if using GROUP BY).COUNT(column_name)
only counts those rows wherecolumn_name
is NOT NULL. This may be slower in some situations even if there are no NULL values because the value has to be checked (unless the column is not nullable).COUNT(1)
is the same asCOUNT(*)
since 1 can never be NULL.
To see the difference in the results you can try this little experiment:
CREATE TABLE table1 (x INT NULL);
INSERT INTO table1 (x) VALUES (1), (2), (NULL);
SELECT
COUNT(*) AS a,
COUNT(x) AS b,
COUNT(1) AS c
FROM table1;
Result:
a b c
3 2 3
COUNT(*) vs. COUNT(1) vs. COUNT(pk): which is better?
Bottom Line
Use either COUNT(field)
or COUNT(*)
, and stick with it consistently, and if your database allows COUNT(tableHere)
or COUNT(tableHere.*)
, use that.
In short, don't use COUNT(1)
for anything. It's a one-trick pony, which rarely does what you want, and in those rare cases is equivalent to count(*)
Use count(*)
for counting
Use *
for all your queries that need to count everything, even for joins, use *
SELECT boss.boss_id, COUNT(subordinate.*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
But don't use COUNT(*)
for LEFT joins, as that will return 1 even if the subordinate table doesn't match anything from parent table
SELECT boss.boss_id, COUNT(*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
Don't be fooled by those advising that when using *
in COUNT, it fetches entire row from your table, saying that *
is slow. The *
on SELECT COUNT(*)
and SELECT *
has no bearing to each other, they are entirely different thing, they just share a common token, i.e. *
.
An alternate syntax
In fact, if it is not permitted to name a field as same as its table name, RDBMS language designer could give COUNT(tableNameHere)
the same semantics as COUNT(*)
. Example:
For counting rows we could have this:
SELECT COUNT(emp) FROM emp
And they could make it simpler:
SELECT COUNT() FROM emp
And for LEFT JOINs, we could have this:
SELECT boss.boss_id, COUNT(subordinate)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
But they cannot do that (COUNT(tableNameHere)
) since SQL standard permits naming a field with the same name as its table name:
CREATE TABLE fruit -- ORM-friendly name
(
fruit_id int NOT NULL,
fruit varchar(50), /* same name as table name,
and let's say, someone forgot to put NOT NULL */
shape varchar(50) NOT NULL,
color varchar(50) NOT NULL
)
Counting with null
And also, it is not a good practice to make a field nullable if its name matches the table name. Say you have values 'Banana', 'Apple', NULL, 'Pears' on fruit
field. This will not count all rows, it will only yield 3, not 4
SELECT count(fruit) FROM fruit
Though some RDBMS do that sort of principle (for counting the table's rows, it accepts table name as COUNT's parameter), this will work in Postgresql (if there is no subordinate
field in any of the two tables below, i.e. as long as there is no name conflict between field name and table name):
SELECT boss.boss_id, COUNT(subordinate)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
But that could cause confusion later if we will add a subordinate
field in the table, as it will count the field(which could be nullable), not the table rows.
So to be on the safe side, use:
SELECT boss.boss_id, COUNT(subordinate.*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
count(1)
: The one-trick pony
In particular to COUNT(1)
, it is a one-trick pony, it works well only on one table query:
SELECT COUNT(1) FROM tbl
But when you use joins, that trick won't work on multi-table queries without its semantics being confused, and in particular you cannot write:
-- count the subordinates that belongs to boss
SELECT boss.boss_id, COUNT(subordinate.1)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
So what's the meaning of COUNT(1) here?
SELECT boss.boss_id, COUNT(1)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
Is it this...?
-- counting all the subordinates only
SELECT boss.boss_id, COUNT(subordinate.boss_id)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
Or this...?
-- or is that COUNT(1) will also count 1 for boss regardless if boss has a subordinate
SELECT boss.boss_id, COUNT(*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
By careful thought, you can infer that COUNT(1)
is the same as COUNT(*)
, regardless of type of join. But for LEFT JOINs result, we cannot mold COUNT(1)
to work as: COUNT(subordinate.boss_id)
, COUNT(subordinate.*)
So just use either of the following:
-- count the subordinates that belongs to boss
SELECT boss.boss_id, COUNT(subordinate.boss_id)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
Works on Postgresql, it's clear that you want to count the cardinality of the set
-- count the subordinates that belongs to boss
SELECT boss.boss_id, COUNT(subordinate.*)
FROM boss
LEFT JOIN subordinate on subordinate.boss_id = boss.boss_id
GROUP BY boss.id
Another way to count the cardinality of the set, very English-like (just don't make a column with a name same as its table name) : http://www.sqlfiddle.com/#!1/98515/7
select boss.boss_name, count(subordinate)
from boss
left join subordinate on subordinate.boss_code = boss.boss_code
group by boss.boss_name
You cannot do this: http://www.sqlfiddle.com/#!1/98515/8
select boss.boss_name, count(subordinate.1)
from boss
left join subordinate on subordinate.boss_code = boss.boss_code
group by boss.boss_name
You can do this, but this produces wrong result: http://www.sqlfiddle.com/#!1/98515/9
select boss.boss_name, count(1)
from boss
left join subordinate on subordinate.boss_code = boss.boss_code
group by boss.boss_name
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