Not a group by expression

SELECT c.cid, p.pid
FROM customers c, orders o, orderline ol, product_info p
WHERE c.cid = o.cid AND o.oid = ol.oid AND ol.pid = p.pid
HAVING(SUM(ol.qty) / (p.sell_qty) > 0.5)
GROUP BY c.cid, p.pid, ol.pid;
 
CREATE VIEW product_info AS
SELECT p.pid, p.pname, p.unit, p.price, p.stock_qty, p.catid, ca.catname, ol.sell_qty
FROM products p
LEFT JOIN categories ca
ON p.catid = ca.catid
LEFT JOIN (SELECT pid, SUM(qty) AS sell_qty FROM orderline GROUP BY pid) ol ON
p.pid = ol.pid;
 
  
CREATE TABLE customers (
  cid           char(5),
  cname         char(15),
  phone         char(12),
  address       char(20),
  PRIMARY KEY (cid)
);
CREATE TABLE categories (
  catid         char(2),
  catname       char(12),
  PRIMARY KEY (catid)
);
CREATE TABLE products (
  pid           char(5),
  pname         char(15),
  unit          char(5),
  price         float,
  stock_qty     integer,
  catid         char(2),
  PRIMARY KEY (pid),
  FOREIGN KEY (catid) REFERENCES categories
);
CREATE TABLE orders (
  oid           char(5),
  cid           char(5),
  odate         date,
  charge_amount float,
  PRIMARY KEY (oid),
  FOREIGN KEY (cid) REFERENCES customers
);
CREATE TABLE orderline (
  lno           integer,
  oid           char(5),
  pid           char(5),
  qty           integer, 
  price         float, 
  PRIMARY KEY (oid, lno),
  FOREIGN KEY (oid) REFERENCES orders ON DELETE CASCADE,
  FOREIGN KEY (pid) REFERENCES products
);