如何 select 表中的某些行但 T-SQL 中 WHOLE 列的平均值？

Question

I have a database of airlines delays and I need to average the delays of their ALL flights by air line but then display only the air lines that fly from city X.

I tried this code:

SELECT  
    B.airline_name,
    AVG(A.arrival_delay) avg_delay
FROM    
    TableDelays A
JOIN    
    TableAirlines B ON A.airline_id = B.airline_id
WHERE 
    A.city = 'X'
GROUP BY 
    B.airline_name

But when I use WHERE Origin = 'X' line, I get incorrect average delay of only the flights that departure from city X. Whereas, when I don't use the WHERE line, I have all air lines with correct averages displayed (from all their flights), but I only need to display the ones from city X.

Does anyone know how to "extract" only the air lines departing from city X so that I don't take it into consideration while averaging the values?

Answer 1

Use a HAVING clause:

SELECT ta.airline_name,
       AVG(td.arrival_delay) as avg_delay
FROM TableDelays td JOIN
     TableAirlines ta
     ON td.airline_id = ta.airline_id
GROUP BY ta.airline_name
HAVING SUM(CASE WHEN td.city = 'X' THEN 1 ELSE 0 END) > 0

Answer 2

Perhaps a CTE would help achieve this?

WITH CTE AS(
    SELECT TA.airline_name,
           AVG(TD.arrival_delay) AS avg_delay,
           MAX(CASE WHEN TD.City = 'X' THEN 1 END) AS XCity
    FROM dbo.TableDelays TD --"A" is a poor choice for an alias here
         JOIN dbo.TableAirlines TA ON TD.airline_id = TA.airline_id --"B" doesn't even appear in "Airlines", why use it?
    GROUP BY TA.airline_name)
SELECT airline_name,
       avg_delay
FROM CTE
WHERE XCity = 1;

As I note in my comments A , and B are poor choices for your table alias. Use suitable ones when giving your tables aliases. Bad habits to kick: using table aliases like (a, b, c) or (t1, t2, t3)