当DISTINCT为默认值时，SELECT查询返回值

Question

I feel this should be a simple SELECT query but cannot get it to work.

Essentially, in this example, I want to return a distinct list of Order_IDs, along with the Country:

• if there is one country associated with the Order_ID, return that Country value

• if there are multiple countries associated, return a default value ('Multiple').

-

SELECT [Order_ID]
    ,CASE WHEN COUNT(DISTINCT [Country]) = 1 THEN [Country] ELSE 'Multiple' END as [Combined_Country]
    FROM [Sales Data]
    GROUP BY [Order_ID]

At the moment this returns the error "Column 'Sales Data.Country' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause."

Adding Country to the GROUP BY clause however results in multiple rows being returned for each country associated with the order, rather than one row with the default value 'Multiple'.

Any help?

Answer 1

You need to aggregate what you aren't grouping. I believe this should work for you:

SELECT 
    [Order_ID]
    , CASE WHEN COUNT(Country) > 1 THEN 'Multiple' ELSE MAX(Country) END AS [Combined_Country]
FROM [Sales Data]
GROUP BY 
    [Order_ID]  

Here, when looking at country, we are always aggregating. The COUNT() aggregation is what you really care about, but we need to use another aggregate function, in my case MAX(), even when trying to read the single value.