如何从python中的两个函数中减去值

Question

I have a two sqlite tables of which both contain different items and their quantities. What I want to do is loop through the first table, select the quantity from the first table and subtract it from the quantity from the second table where the item names match.

(Table1)

items  | quantity
----------
sugar  | 50
Pen    | 10
apple  | 50
berry  | 1

Second Table(populated by user input)(Table 2)

items  |quantity
----------
sugar  |500
Pen    |5
apple   |8
berry  |1

So as I said, I want to loop through table 1 and get the quantity of sugar from the Table 1. Then loop through Table 2 and get the quantity of sugar in table 2 too, then finally subtract the values and return the value after the subtraction has been carried out. And then the same thing is done for the other items in the table1.(loop)

Eg Using sugar, (Quantity of Sugar from table 2 – Quantity of Sugar from table 1) (500 – 50) = 450 Then it returns the value 450 so I can use it in other functions

Code Example.

import sqlite3

db = sqlite3.connect('example.db')
#db.execute('create table Table1 (items text, quantity integer)')
#db.execute('create table Table2 (items text, quantity integer)')
a = 'Sugar'
b = 50
c = 500
db.execute('insert into Table1 (items, quantity) values(?,?)',(a, b))
db.execute('insert into Table1 (items, quantity) values(?,?)',(a, c))
db.commit()
db.close()

def one():
    cursor = db.execute('select items from Table1')
    for i in cursor.fetchall():
        return i

def two():
    cursor1 = db.execute('select quantity from Table1 where items = ?',(one()))
    for i in cursor1.fetchall():
        return i

def three():
    cursor1 = db.execute('select quantity from Table2 where items = ?',(one()))
    for i in cursor1.fetchall():
        return i

def subtraction():
    t = three() - two()
    return t

The error shown when i run the above code File "C:/Users/MegaMind/Documents/Programming/Pycham Projects/New folder/package.py", line 22, in subtraction t = three() - two() TypeError: unsupported operand type(s) for -: 'tuple' and 'tuple'

If there is a cleaner way to do this, please assist in any way you can.

Answer 1

You can simply tell the database to join the two tables:

cursor = db.execute('''SELECT items, Table2.quantity - Table1.quantity
                       FROM Table1
                       JOIN Table2 USING (items)''')
for row in cursor:
    name = row[0]
    diff = row[1]
    ...

Answer 2

You're making it much too hard. SQL can do that by itself and give you the results in a silver platter:

SELECT table1.items, table1.quantity - table2.quantity as remaining 
FROM table1 LEFT JOIN table2 ON table1.items = table2.items

The LEFT JOIN ensures that all rows of table1 will appear in the result. If you only want the reduced counts, use plain JOIN .