Selecting data from a sub query

Question

I am doing the exercises this is the link (https://www.wiseowl.co.uk/sql/exercises/standard/subqueries/4228/). Can anyone help me. Thanks in advance!

Question

Write a SELECT statement to return events from the 3 continents with the fewest events.

Help

To do this first write a SELECT query which returns all the continents and events.

Subquery select

Unsorted results returns 459 events.

Now underneath write another SELECT statement which lists events for the 3 continents with the lowest COUNT of events. Put the COUNT in the ORDER BY clause, not the SELECT.

Finally use the second SELECT as a filter in the first SELECT's WHERE clause. To do this use ContinentName IN (subquery).

Subquery Continent

Only 8 events in these 3 continents - maybe stick to Ibiza and the Islands!

My query

Select [ContinentName], [EventName] 
from tblevent a 
join[dbo].[tblCountry] c on a.[CountryID]=c.[CountryID] 
join [dbo].[tblContinent] b on c.[ContinentID]=b.[ContinentID]
where [ContinentName] in
(
  select top 3 [ContinentName] 
  from tblevent a join[dbo].[tblCountry] c  on a.[CountryID]=c.[CountryID] 
  join [dbo].[tblContinent] b on c.[ContinentID]=b.[ContinentID]
  group by [ContinentName] 
) 
order by count([EventName])

Result:

Error message when I run above query:

Column 'dbo.tblContinent.ContinentName' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

The problem goes away if I remove the order by count([EventName]) but I need events from 3 continents with the fewest events, so order by count([EventName]) must be there. There is some other problem causing the error.

Answer 1

The error is due to using COUNT() in a query without GROUP BY - which means GROUP BY () - and at the same time having some columns in the SELECT list. You need to move that COUNT() in the subquery.

Assuming you want to show all Events from the 3 continents with lowest count of events:

Select c.[ContinentName], a.[EventName] 
from tblevent a 
     join [dbo].[tblCountry] c
       on a.[CountryID] = c.[CountryID] 
     join [dbo].[tblContinent] b
       on c.[ContinentID] = b.[ContinentID]
where [ContinentName] in
(
  select top 3 [ContinentName]
  from tblevent a 
       join [dbo].[tblCountry] c
         on a.[CountryID] = c.[CountryID] 
       join [dbo].[tblContinent] b
         on c.[ContinentID] = b.[ContinentID]
  group by [ContinentName]
  order by count(*) 
)  ;