sql Alias Column Name for Use in CASE Statement - sql-server-2008

Question

I have an sql query as below,
select Site,DataSource,
(SELECT CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END) AS OperatorScenario,
(SELECT CASE
WHEN OperatorScenario = 'ChangeOperator' THEN '1'
ELSE '022'
END) AS OperatorScenario2
from tablename
when i execute this query i am getting exception as Invalid column name OperatorScenario. So i tried to use single codes in alias name in second query as below,
(SELECT CASE
WHEN 'OperatorScenario' = 'ChangeOperator' THEN '1'
ELSE '022'
END) AS OperatorScenario2
So then it executes always else part. Please give me some suggesssions.
Regards
sangeetha

Answer 1

The main issue is you cannot reference an alias in the same query. Also you have parentheses and an extra SELECT keyword, making what looks like a subquery where I don't believe you intended one (you do not have a FROM clause).
You can either copy the whole thing again (fixing your query in the process):
SELECT Site, DataSource,
CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END AS OperatorScenario,
CASE
WHEN
CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END = 'ChangeOperator' THEN '1'
ELSE '022'
END AS OperatorScenario2
from tablename
Or, simplifying:
SELECT Site, DataSource,
CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END AS OperatorScenario,
CASE
WHEN DataSource = 'RFQ' THEN '1'
ELSE '022'
END AS OperatorScenario2
from tablename
Or use a CTE (again fixing your query):
WITH cte AS
(
SELECT Site, DataSource,
CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END AS OperatorScenario,
FROM tablename
)
SELECT Site, DataSource, OperatorScenario,
CASE
WHEN OperatorScenario = 'ChangeOperator' THEN '1'
ELSE '022'
END AS OperatorScenario2
FROM cte

Answer 2

You cannot access the alias on the same level in this query, either use a sub-query or a cte:
WITH cte
AS (SELECT site,
datasource,
CASE
WHEN datasource = 'RFQ' THEN 'ChangeOperator'
ELSE 'SameOperator'
END AS OperatorScenario
FROM dbo.tablename)
SELECT site,
datasource,
operatorscenario,
CASE
WHEN operatorscenario = 'ChangeOperator' THEN '1'
ELSE '022'
END AS OperatorScenario2
FROM cte
Note that i've also removed the extra parantheses and select in the CASE.

Answer 3

I set my aliases at the beginning like this:
SELECT [ALIAS] = CASE....
Yours would be:
SELECT Site, DataSource,
[OperatorScenario] = CASE
WHEN DataSource = 'RFQ' THEN 'ChangeOperator' ELSE 'SameOperator'
END,
[OperatorScenario2] = CASE
WHEN DataSource = 'RFQ' THEN '1' ELSE '022'
END
from tablename