Question

I have a rather complex logical query I’m trying to execute. Essentially there are a series of related tables. I am having difficulty figuring out a good way to approach it.

Tables:

Transaction –
T_id

Discount –
D_id
item_id
Type (percentage or basic value)
value

Transaction_Discount –
T_id
D_id

Item –
item_id
price

EDIT: additional table
Purchase
T_id
item_id

I am attempting to pull the price adjusted after the discount is applied. I would like to avoid adding another column with the adjusted price to the database… as it is possible to calculate the adjusted price for each transaction from the data already entered (so it’d be redundant to store this data).

This is the basic logic:

if(this transaction had a discount applied)  
  //apply the discount and return the adjusted price  
else(no discount applied)  
  //pull price

It is possible to do this in several separate queries using PHP logic.

//1st step - create an array of bool's: true = discount_used/false = no_discount
//2nd step - if(discount_used) return price adjusted to discount
//3rd step - if(no_discount) return price
//4th step - combine the two different arrays

this is clearly very bulky. It seems like there HAS to be a better way to do this that is likely much more efficient.

It is my understanding that you can perform queries that contain logic in mysql, would that help here?

Answer 1

Lets say that you have an item_id on the Transactions Table, and that Percentage is represented by ‘P’, you could try something like

SELECT  *,
        CASE 
            WHEN td.T_id IS NULL
                THEN it.price
            WHEN td.T_id IS NOT NULL AND d.Type = 'P'
                THEN id.price - (id.price * d.value)
            ELSE id.price - d.value
        END PriceDiscounted

FROM    Transaction t LEFT JOIN
        Purchase p ON t.T_id = p.T_id LEFT JOIN
        Item it ON p.item_id = it.item_id LEFT JOIN
        Transaction_Discount td ON t.T_id = td.T_id LEFT JOIN
        Discount d ON td.D_id = d.D_id LEFT JOIN
        Item id ON d.item_id = id.item_id