SQL SERVER - Using DATEDIFF with subquery column

Question

I have a query that looks like:

SELECT
    col1
    ,...
    ,col3
    ,(SELECT col3 FROM table where <clause>) AS MinPickTime
    ,(SELECT col3 FROM table where <clause>) AS MaxPickTime
    ,DATEDIFF(d, MinPickTime, MaxPickTime)
FROM table

However the DATEDIFF line does not like the alias columns.

In short, how do I give DATEDIFF an alias column derived by a subquery?

Answer 1

Use derived tables concept to access alias name.

SELECT  col1
    ,...
    ,col3,
     MinPickTime,
     MaxPickTime ,
DATEDIFF(d, MinPickTime, MaxPickTime)
FROM (
SELECT
    col1
    ,...
    ,col3
    ,(SELECT col3 FROM table where <clause>) AS MinPickTime
    ,(SELECT col3 FROM table where <clause>) AS MaxPickTime

FROM table
)z

Answer 2

with temp as (SELECT
    col1
    ,...
    ,col3
    ,(SELECT col3 FROM table where <clause>) AS MinPickTime
    ,(SELECT col3 FROM table where <clause>) AS MaxPickTime
FROM table)

select DATEDIFF(d,MaxPickTime,MinPickTime) from temp