I am a little code blind today because i am trying to write this simple query that should select all the books and show their prices. The problem is that prices is stored in the book\_sales, and when i use that one it will show all time the books has been sold. Therefor i want to group them by identifiers, so i can only see: 

* BookTitle1 - 99,- - BookID923109

But now i see the same one repeating over and over. 

* BookTitle1 - 99,- BookID923109
* BookTitle1 - 99,- BookID923109
* BookTitle1 - 99,- BookID923109

Tried to resolve with this query: 

```
select title,sales_price,identifiers
from BOOKS
INNER JOIN BOOK_SALES
on BOOKS.book_id = BOOK_SALES.book_id
GROUP BY IDENTIFIERS

```

i get "[Err] 42000 - [SQL Server]Column 'BOOKS.TITLE' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause."

whats wrong fellas?