How to use the column name alias from a case statement to a join statement

Question

How to use the column name alias from a case statement to a join statement

SQL_Hunt

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Points: 33518
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December 20, 2020 at 4:44 am

#3822493

Hello
While trying to use the case alias column in the join statement, getting the invalid column error.
Error msg: Invalid column name 'ST100'.
SELECT TOP (1000)
CASE
WHEN [Sx].ac_No IN (
'ARC'
,'GFT'
,'DHG'
,'STF'
,'FRX'
)
THEN '45010'
ELSE CD.Idea
END AS [ST100]
FROM CONSUL AS CD
LEFT JOIN table2 AS [Sx] ON CD.clumn1 = [Sx].column1
LEFT JOIN table3 AS [Fx] ON ST100 = Fx.texo
Is there anyway to address this need?

Thanks.

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Lowell SSC Guru Points: 323504 More actions · Answer 1

To leverage the Alias, I think you would need to either use a common Table Expression or a sub query to expose the alias to the next statements.

Alais's are evaluated AFTER joins, but nesting them in sub queries will do what you are asking, here's an example.

WITH MyCTE
AS
(
SELECT TOP (1000)
CASE 
WHEN [Sx].ac_No IN (
'ARC'
,'GFT'
,'DHG'
,'STF'
,'FRX'
)
THEN '45010'
ELSE CD.Idea
END AS [ST100]
FROM CONSUL AS CD
)
SELECT CD.*,Sx.*,Fx.*
FROM MyCTE AS CD
LEFT JOIN table2 AS [Sx] ON CD.clumn1 = [Sx].column1

LEFT JOIN table3 AS [Fx] ON ST100 = Fx.texo

Lowell

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jcelko212 32090 SSCrazy Eights Points: 9294 More actions · Answer 2

Look up the order in which the clauses in a select statement are executed in SQL. The actual SELECT list is the last one performed, so your alias doesn't exist while the other clauses are being executed. You can do some nesting (subqueries, CTE, etc.) to materialize the results you want. Then you can reference them. The basic scoping rules in SQL are something like any block structured language, but not quite.

Please post DDL and follow ANSI/ISO standards when asking for help.

ScottPletcher SSC Guru Points: 100947 More actions · Answer 3

In SQL Server, there is a way to assign a usable column alias, using CROSS APPLY:

SELECT TOP (1000) ST100
FROM CONSUL AS CD
LEFT JOIN table2 AS [Sx] ON CD.clumn1 = [Sx].column1
CROSS APPLY (
    SELECT 
    CASE
    WHEN [Sx].ac_No IN (
    'ARC'
    ,'GFT'
    ,'DHG'
    ,'STF'
    ,'FRX'
    )
    THEN '45010'
    ELSE CD.Idea
    END AS [ST100]
) AS alias1
LEFT JOIN table3 AS [Fx] ON ST100 = Fx.texo

You can even embed / stack CROSS APPLY alias references, for example:

SELECT *

FROM ( SELECT 1 AS a, 2 AS b ) AS main_data

CROSS APPLY ( SELECT a + b AS c ) AS alias1

CROSS APPLY ( SELECT c + 5 AS d ) AS alias2_using_alias1

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