SQL Anywhere error - Function or column reference to 'id' must also appear in a GROUP BY

SQL Anywhere error - Function or column reference to 'id' must also appear
in a GROUP BY

I have 2 tables:
test table:
Column Type Nullable
Primary Key
-------------------------------- -------------------------------- --------
-----------
id integer 0
1
name varchar(255) 1
0
price varchar(255) 1
0
rows:
id name price
1 BMW 3000
2 AUDI 2500
test_2 table:
Column Type Nullable
Primary Key
-------------------------------- -------------------------------- --------
-----------
id integer 0
1
ad_id integer 1
0
user_id integer 1
0
price integer 1
0
rows:
id ad_id user_id price
----------- ----------- ----------- -----------
1 1 23 2000
2 1 23 2200
3 1 22 2050
I need to get this result:
BMW 23 2200
BMW 22 2050
My SQL:
SELECT * FROM test, test_2 WHERE test_2.ad_id=test.id GROUP BY user_id
ORDER BY test_2.price DESC
but always I get this error:
Could not execute statement.
Function or column reference to 'id' must also appear in a GROUP BY
SQLCODE=-149, ODBC 3 State="42000"
Line 1, column 1
I use SQL Anywhere. Thanks for any help.