How to use a column created using case statement to calculate another field value in select

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Hi Guys,

I have a query that I'm creating a column in select using case statement

SQL

select col1, (case when t1.org='x' then xDate else yDate) as newDate,(case when newDate< joiningDate then 1 else 0) as PDate
from table1 t1
left join table2 on t1.id=t2.id

I'm unable to find the column newDate in IntelliSense in order to create column PDate. if I type it manually it throws an invalid column error.
I'm not sure how exactly does it work.

Can anyone help me to get the desired result?

Thank you.

What I have tried:

I'm not sure how does it work.

Posted 19-Sep-21 22:54pm

PreetMDX

Updated 20-Sep-21 3:01am

Richard Deeming

v2

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2 solutions

Solution 2

Thank you Richard, Cross Apply just worked great :) and got to know a use case as well.

Posted 20-Sep-21 3:01am

PreetMDX

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Richard Deeming · Accepted Answer · 2021-09-19T23:33:00

Calculated columns aren't available in the same SELECT statement where they're defined. You either need to repeat the calculation:

SQL

CASE 
    WHEN t1.org = 'x' And xDate < joiningDate THEN 1
    WHEN t1.org != 'x' And yDate < joiningDate THEN 1
    ELSE 0
END As PDate

Or you can use CROSS APPLY to define your calculated column:

SQL

SELECT
    col1,
    D.newDate,
    CASE WHEN D.newDate < joiningDate THEN 1 ELSE 0 END As PDate
FROM
    table1 As t1
    LEFT JOIN table2 As t2 ON t1.id = t2.id
    CROSS APPLY (SELECT CASE WHEN t1.org = 'x' THEN xDate ELSE yDate END As newDate) As D
;

The many uses of CROSS APPLY | SQL Studies[^]