An aggregate may not appear in the OUTPUT clause.

Error Message:
Msg 158, Level 15, State 1, Line 2
An aggregate may not appear in the OUTPUT clause.

Severity level:
15.

Description:
This error message appears when you try to use an aggregate function in the OUTPUT clause of an UPDATE statement.

Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.

Resolution:
Error of the Severity level 15 are generated by the user and are corrigible by the user. Aggregate functions are not allowed in the OUTPUT clause.

Versions:
This error message was introduced with SQL Server 2005.

Example(s):
USE tempdb;
GO
IF OBJECT_ID('tempdb..#t') > 0
 DROP TABLE #t
GO
CREATE TABLE #t
(
 c1 INT
);
GO
INSERT INTO #t SELECT 1;
UPDATE #t SET c1 = 2
OUTPUT SUM(inserted.c1);

Remarks:
In the above example we try to use the SUM() aggregate function in the OUTPUT clause of an UPDATE statement. This raises the error.