Question

select 
    case location_id 
      when 1 then 'DELHI' when 2 then 'AHMEDABAD' 
      when 4 then 'HYDERABAD' when 5 then 'KOLKATA' when 6 then 'BANGALORE'
      when 7 then 'MUMBAI' when 8 then 'CHENNAI' 
   end as CITY,
   count(*) as Total
from #tmptab1
group by CITY

I get error

Msg 207, Level 16, State 1, Line 12
Invalid column name ‘CITY’

How to rectify this ? Please help.

Answer 1

You can’t use column aliases in GROUP BY clauses. Please see Conceptual Order of Evaluation of a Select Statement, where you will see that the clauses are (logically) evaluated in this order: FROM, WHERE, GROUP BY, HAVING, SELECT, DISTINCT, UNION, ORDER BY.

That is not exactly how the engine performs the operation, but it is a useful heuristic for a solid practical understanding of why you can’t use something from the SELECT statement in the GROUP BY clause — it isn’t logically available to the GROUP BY clause as it is evaluated later.

There are several ways around this:

1. Repeat the entire expression in the GROUP BY clause:

   SELECT
      City =
         CASE location_id when 1 then 'DELHI' when 2 then 'AHMEDABAD' 
         when 4 then 'HYDERABAD' when 5 then 'KOLKATA' when 6 then 'BANGALORE'
         when 7 then 'MUMBAI' when 8 then 'CHENNAI'
         END,
      Total = Count(*)
   FROM #tmptab1
   GROUP BY
      CASE location_id when 1 then 'DELHI' when 2 then 'AHMEDABAD' 
      when 4 then 'HYDERABAD' when 5 then 'KOLKATA' when 6 then 'BANGALORE'
      when 7 then 'MUMBAI' when 8 then 'CHENNAI'
      END
   

2. Use a derived table:

   SELECT
      City,
      Total = Count(*) 
   FROM
      (
         SELECT
            City =
               CASE location_id when 1 then 'DELHI' when 2 then 'AHMEDABAD' 
               when 4 then 'HYDERABAD' when 5 then 'KOLKATA' when 6 then 'BANGALORE'
               when 7 then 'MUMBAI' when 8 then 'CHENNAI'
               END
         FROM #tmptab1
      ) Cities
   GROUP BY City;
   

3. Use a Common Table Expression (CTE), SQL Server 2005 and up:

   WITH Cities AS (
       SELECT
          City =
             CASE location_id 
                when 1 then 'DELHI' 
                when 2 then 'AHMEDABAD' 
                when 4 then 'HYDERABAD' 
                when 5 then 'KOLKATA' 
                when 6 then 'BANGALORE'
                when 7 then 'MUMBAI' 
                when 8 then 'CHENNAI'
             END
       FROM #tmptab1
   )
   SELECT
      City,
      Total = Count(*) 
   FROM Cities
   GROUP BY City;
   

4. Use CROSS APPLY, SQL Server 2005 and up:

   SELECT
      City,
      Total = Count(*)
   FROM
      #tmptab1
      CROSS APPLY (
         SELECT
            City =
               CASE location_id when 1 then 'DELHI' when 2 then 'AHMEDABAD' 
               when 4 then 'HYDERABAD' when 5 then 'KOLKATA' when 6 then 'BANGALORE'
               when 7 then 'MUMBAI' when 8 then 'CHENNAI'
               END
      ) C
   GROUP BY City;
   

Since your expression is deterministic, it is possible that you could simply do GROUP BY location_id, however this is not the normal case, and you shouldn’t go around expecting to be able to circumvent normal aggregate grouping logic by selecting a single column, because most of the time such a CASE expression adds value that is not deterministic.

In fact, because the information is not only deterministic but is about the real world (rather than the business rules) I would recommend that you NOT encode this information in your query! Create a Location table and join to it. It is not best practice to put changeable user data directly into queries–queries are supposed to record process, not content, and what if you add a new location_id? All the queries that use it will have to change. Additionally, grouping by location_id will not work properly if more than one location_id can refer to the same city.