Question

I’m trying to build a query with 4 different tables:

• Users_Licences
• Licences
• Products
• Request

The Users have multiple Licences, stored in Users_Licences. Products have multiple Requests and in the Request table you have target licence where in the Products table you have source_licence.

I want to list all the requests that have target licences and source licences (via Products) that the specified user has in his Users_Licences table.

Complicated but this is where I’m at this point (not working):

SELECT 
    Request.id AS offer_id, Request.product_id, 
    Request.licence_id AS target_licence_id, 
    Request.trans_price, Products.source_licence_id    
FROM 
    Request    
JOIN 
    Products ON Request.product_id = Products.id    
JOIN 
    Licences ON Products.source_licence_id = Licences.id    
JOIN 
    Licences ON Request.licence_id = Licences.id    
JOIN 
    Users_Licences ON Licences.id = Users_Licences.licence_id    
WHERE 
    Users_Licences.user_id = '$user_id'    
GROUP BY 
    Request.id    
ORDER BY 
    Request.trans_price DESC

Could you help me out? If I delete the line

JOIN Licences ON Products.source_licence_id = Licences.id

it works but I have part of my result…Thanks!

Answer 1

You need to use different alias name wheh you use the table more than a time

SELECT Request.id AS offer_id, Request.product_id, Request.licence_id AS target_licence_id, Request.trans_price 
, Products.source_licence_id     
FROM Request     
JOIN Products ON Request.product_id = Products.id     
JOIN Licences as l1 ON Products.source_licence_id = l1.id     
JOIN Licences as l2 ON Request.licence_id = l2.id     
JOIN Users_Licences ON l2.id = Users_Licences.licence_id     
WHERE Users_Licences.user_id='$user_id'     
GROUP BY Request.id     
ORDER BY Request.trans_price DESC