COUNT
Returns the number of items in a group.
Syntax
COUNT ( { [ ALL | DISTINCT ] expression ] | * } )
Arguments
ALL
Applies the aggregate function to all values. ALL is the default.
DISTINCT
Specifies that COUNT returns the number of unique nonnull values.
expression
Is an expression of any type except uniqueidentifier, text, image, or ntext. Aggregate functions and subqueries are not permitted.
*
Specifies that all rows should be counted to return the total number of rows in a table. COUNT(*) takes no parameters and cannot be used with DISTINCT. COUNT(*) does not require an expression parameter because, by definition, it does not use information about any particular column. COUNT(*) returns the number of rows in a specified table without eliminating duplicates. It counts each row separately, including rows that contain null values.
Important Distinct aggregates, for example AVG(DISTINCT column_name), COUNT(DISTINCT column_name), MAX(DISTINCT column_name), MIN(DISTINCT column_name), and SUM(DISTINCT column_name), are not supported when using CUBE or ROLLUP. If used, Microsoft® SQL Server™ returns an error message and cancels the query.
Return Types
int
Remarks
COUNT(*) returns the number of items in a group, including NULL values and duplicates.
COUNT(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.
COUNT(DISTINCT expression) evaluates expression for each row in a group and returns the number of unique, nonnull values.
Examples
A. Use COUNT and DISTINCT
This example finds the number of different cities in which authors live.
USE pubs
GO
SELECT COUNT(DISTINCT city)
FROM authors
GO
Here is the result set:
-----------
16
(1 row(s) affected)
B. Use COUNT(*)
This example finds the total number of books and titles.
USE pubs
GO
SELECT COUNT(*)
FROM titles
GO
Here is the result set:
-----------
18
(1 row(s) affected)
C. Use COUNT(*) with other aggregates
The example shows that COUNT(*) can be combined with other aggregate functions in the select list.
USE pubs
GO
SELECT COUNT(*), AVG(price)
FROM titles
WHERE advance > $1000
GO
Here is the result set:
----------- --------------------------
15 14.42
(1 row(s) affected)