sql - how to approach this specific query -

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i have rather complex logical query i'm trying execute. there series of related tables. having difficulty figuring out way 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 attempting pull price adjusted after discount applied. avoid adding column adjusted price database... possible calculate adjusted price each transaction data entered (so it'd redundant store data).

this basic logic: 

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

it possible in several separate queries using php logic.

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

this bulky. seems there has better way more efficient.

it understanding can perform queries contain logic in mysql, here?

lets have item_id on transactions table, , percentage represented 'p', try like

select  *,         case              when td.t_id null                 it.price             when td.t_id not null , d.type = 'p'                 id.price - (id.price * d.value)             else id.price - d.value         end pricediscounted     transaction t left join         purchase p on t.t_id = p.t_id left join         item 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

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