An aggregate cannot appear in an ON clause unless it is in a subquery contained in a HAVING clause or select list, and the column being aggregated is an outer reference.

Error Message:
Msg 1015, Level 15, State 1, Line 1
An aggregate cannot appear in an ON clause unless it is in a subquery contained in a HAVING clause or select list, and the column being aggregated is an outer reference.

Severity level:
15.

Description:
This error message appears when you try to use an aggregate function in the ON clause of a join.

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

Resolution:
Errors of the Severity Level 15 are generated by the user and are corrigible by the user. The statement cannot be executed this way. The aggregate function cannot be used this way.

Versions:
All versions of SQL Server.

Example(s):
SELECT *
  FROM Northwind.dbo.Customers c
  JOIN Northwind.dbo.Orders o
    ON COUNT(c.city) < o.OrderID

Remarks:
One possible alternative would be the use of the aggregate function in the HAVING clause.