Gør nogle ret basale summer med forskellige tabeller

En tilgang ville være at bruge inline-visninger. For at gøre dette

• Opret en forespørgsel, der summerer de optjente point
• Opret en forespørgsel, der summerer de brugte point
• Sæt forespørgslerne sammen (en venstre joinforbindelse, hvis de ikke har brugt noget endnu)
• Foretag din subtraktion

SELECT SumOfPointsEarned.Points - COALESCE(SumOfPointsOfPurchasesMade.Points, 0) AS CurrentPoints
FROM   
(
    SELECT SUM(Points) AS Points, Recipient_ID 
    FROM   transactions 
            WHERE Recipient_ID= 137642
    GROUP  BY Recipient_ID
) AS SumOfPointsEarned 
    LEFT JOIN 
    (
        SELECT purchases.Student_ID,  SUM(rewards.Cost_to_User) AS Points 
        FROM   purchases 
            INNER JOIN rewards 
            ON purchases.Reward_ID = rewards.Reward_ID 
                    WHERE Student_ID = 137642
        GROUP  BY purchases.Student_ID
    ) AS SumOfPointsOfPurchasesMade 
    ON SumOfPointsEarned.Recipient_ID = SumOfPointsOfPurchasesMade.Student_ID