Stačí se připojit k tabulce2 třikrát filtrováním pro var_name v klauzuli JOIN.
SELECT t.parent_id,
t.id,
t.name,
t.quality,
t.price,
c.VALUE AS color,
s.VALUE AS size,
r.VALUE AS rating
FROM table1 t
LEFT JOIN table2 c
ON t.parent_id = c.parent_id
AND c.var_name = 'color'
LEFT JOIN table2 s
ON t.parent_id = s.parent_id
AND s.var_name = 'size'
LEFT JOIN table2 r
ON t.parent_id = r.parent_id
AND r.var_name = 'rating'