计数 (*) 与计数 (1) - SQL Server [英] Count(*) vs Count(1) - SQL Server

问题描述

Just wondering if any of you people use Count(1) over Count(*) and if there is a noticeable difference in performance or if this is just a legacy habit that has been brought forward from days gone past?

The specific database is SQL Server 2005.

解决方案

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 the <value expression> to each row of T and eliminating null values. If one or more null values are eliminated, then a completion condition is raised: warning-

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