I have two tables in database called `purchase` AND `sales`
Tables as follow

Table `purchases` 

```
----------------------------
ID  PRODUCT   QTY 
----------------------------
1   APPLE     100
2   BANANA    200
3   GRAPES    150
4   AVACADO   110

```

Table `Sales`

```
----------------------------
ID  PRODUCT   QTY 
----------------------------
1   APPLE     50
2   BANANA    200
3   GRAPES    100

```

Here in the above tables i have `purchase` and `sales` i want to do subtraction from `purchases` quantity with `sales` quantity

and get the output like this 

```
Output
----------------------------
ID  PRODUCT   QTY 
----------------------------
1   APPLE     50
2   BANANA    0
3   GRAPES    50
4   AVACADO   110

```

i written query but not working 

```
select id, product, (purchases.qty)-(sales.qty) from purchases left join sales on purchases.id=sales.id

```