计算所有列的平均值，然后计算输出差异

Question

Hello i am having some trouble determining what is wrong with my query.

I am trying to output the average cost of all events unrelated to the Event name. However when processing my query a connection seems to be made between the event name and the average. I believe the query is generating the average cost of each event related to that name not the total average. Once i have the average i want to then display only the events that cost beyond the average cost price.

Query:

SELECT Event_Name AS 'Event Name'
              ,ROUND(Event_Cost) AS 'Event Cost'
              ,ROUND(AVG(Event_Cost)) AS 'Average Cost Of Events'
              ,ROUND(Event_Cost - AVG(Event_Cost)) AS 'Amount Over Average £'
FROM Event
WHERE Event_Cost > (SELECT AVG(Event_Cost) FROM Event)
GROUP BY Event_Name;

I am sure it is something really simple i am missing. Thanks in advance

Answer 1

There are two problems in your query. The first one is that since you're selecting the event_name, your average is calcluated at that level; the other problem is that you're using the Event_Cost column both as is and aggregated at the same time, from the same table, and that's not possible.

You should get the aggregated value from an inner query and cross join it with your initial table, like this

SELECT  t1.Event_Name AS 'Event Name',
        ROUND(Event_Cost) AS 'Event Cost',
        ROUND(t2.aver) AS 'Average Cost Of Events',
        ROUND(Event_Cost - t2.aver) AS 'Amount Over Average £'
FROM    Event t1
CROSS JOIN
        (
            SELECT  AVG(Event_cost) as aver
            FROM    Event
        ) t2
WHERE   t1.Event_Cost > t2.aver;