How to reference column by its alias within ROW_NUMBER

Question

I'm told that ROW_NUMBER is the best way to select the top row from each group, but it doesn't work. Here is my example

SELECT
    TOP 10000
    CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END AS PosDiag,
    var1, var2, var3, dat4,
    ROW_NUMBER() OVER (PARTITION BY var1 ORDER BY PosDiag DESC) rn 
FROM
    mydb 
ORDER BY var1, PosDiag DESC, dat4 ASC

This just throws an error:

Invalid column name 'PosDiag'

If I remove PosDiag from the ORDER BY it still throws the error. At some point I have to order by both var1 and then PosDiag to bring the PosDiag = 'Y' to the top of each group, when/where it exists, so that the newly created row number will always correspond to row number 1 and can later be extracted.

Answer 1

You can't reference aliases defined in the SELECT clause in other alias definitions, etc.

You need to define the alias in a sub-query in order to use it in other expressions.

SELECT
  TOP 10000
  posDiag,
  var1, var2, var3, dat4,
  ROW_NUMBER() OVER (PARTITION BY var1 ORDER BY PosDiag DESC) rn 
FROM
(
  SELECT
    mydb.*,
    CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END AS PosDiag
  FROM
    mydb
)
  mydb 
ORDER BY
  var1,
  PosDiag DESC,
  dat4 ASC

Also, bear in mind that SQL is a Declarative Language. You write an expression and then it is parsed and 'compiled' in to an execution plan. Just because you have a sub-query to fit the language doesn't mean that the sub-query adds un-necessary processing.

EDIT:

Alternative forms...

WITH
  mydb AS
(
  SELECT
    mydb.*,
    CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END AS PosDiag
  FROM
    mydb
)
SELECT
  TOP 10000
  posDiag,
  var1, var2, var3, dat4,
  ROW_NUMBER() OVER (PARTITION BY var1 ORDER BY PosDiag DESC) rn 
FROM
  mydb 
ORDER BY
  var1,
  PosDiag DESC,
  dat4 ASC

or...

SELECT
  TOP 10000
  posDiag,
  var1, var2, var3, dat4,
  ROW_NUMBER() OVER (PARTITION BY var1 ORDER BY PosDiag DESC) rn 
FROM
  mydb
CROSS APPLY
(
  SELECT CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END
)
  AS translate_var3(PosDiag)
ORDER BY
  var1,
  PosDiag DESC,
  dat4 ASC

Answer 2

After you set the value for PosDial, try replacing it (PosDiag) with its content (CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END), as it is an alias and its value is not known within the same query:

SELECT
    TOP 10000
    CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END AS PosDiag,
    var1, var2, var3, dat4,
    ROW_NUMBER() OVER (PARTITION BY var1 ORDER BY  CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END DESC) rn 
FROM
    mydb 
ORDER BY var1,  CASE WHEN var3 IN ('string1', 'string2') THEN 'Y' ELSE 'N' END DESC, dat4 ASC