Question

I seem to be having a problem with my SQL query that’s driving me mad. I just can’t seem to get the group by to work. I keep getting the following error:

00979. 00000 -  "not a GROUP BY expression"

My query:

SELECT customers.customer_first_name, customers.customer_last_name, orders.customer_numb, books.author_name, books.title
FROM customers
LEFT OUTER JOIN orders ON (orders.customer_numb = customers.customer_numb)
LEFT OUTER JOIN order_lines ON (order_lines.order_numb = orders.order_numb)
LEFT OUTER JOIN books ON (books.isbn = order_lines.isbn)
WHERE (customers.customer_numb = 6)
GROUP BY (books.title)

Database schema:

Customers:

Order Lines & Orders:

What I’m trying to achieve:
I’m trying to group by book titles so that it doesn’t show duplicate titles.

I apologise if I’ve missed anything, thanks.

Answer 1

What don’t you understand? Your select statement contains many columns that are not in the group by clause. Try this:

SELECT customers.customer_first_name, customers.customer_last_name, orders.customer_numb, books.author_name, books.title 
FROM customers 
LEFT OUTER JOIN orders ON (orders.customer_numb = customers.customer_numb) 
LEFT OUTER JOIN order_lines ON (order_lines.order_numb = orders.order_numb) 
LEFT OUTER JOIN books ON (books.isbn = order_lines.isbn) 
WHERE (customers.customer_numb = 6) 
GROUP BY customers.customer_first_name, customers.customer_last_name, orders.customer_numb, books.author_name, books.title  

Since you are not aggregating anything, you can dispense with the group by, and just use distinct in the select clause:

select distinct  customers.customer_first_name, customers.customer_last_name, orders.customer_numb, books.author_name, books.title