Question

I have a table (foos) which is a list of Foos, one row per type of Foo. A second table (items) is a list in which each row is the type of Foo and an amount (and other information). For example, Foo3, 45.2 and Foo2, 12.34.

I’d like to determine the total of the amounts for each type of Foo.

This is my existing code, but there must be a better (more standard or efficient) way:

cursor.execute('''select type from foos''')
foo_types = cursor.fetchall()
results = []
for ft in foo_types:
    cursor.execute('''select sum(amount) from items
        where foo_type =?''', ft)
    results.append((ft, cursor.fetchone()))

How should I code this?

Answer 1

SELECT foo_type, SUM(amount)
FROM items
GROUP BY foo_type

gives you within one query already each foo_type and the corresponding sum. You can build a dictionary from it and use it to expand data from the first query.

Or put everything into one query:

cursor.execute("SELECT foo_type, SUM(amount) "
               "FROM items, foos "
               "WHERE items.foo_type = foos.type "
               "GROUP BY foo_type")
results = list(cursor)

# results is a list of tuples: [(type1, sum1), (type2, sum2), ...]