Expression in WHERE and SELECT

• loki1049

  SSCrazy

  Points: 2065

  More actions

  November 18, 2010 at 4:23 pm

  #241410

  I know that you can't use an ALIAS in the WHERE clause, but if I were to make an expression and use it as a filter in the WHERE clause and I also want to return it as an item of the SELECT list does it get calculated twice?

  Heres a hypothetical example:

  SELECT

  People.FirstName,

  People.LastName,

  WWII_Status = (CASE WHEN People.Birthdate <= '1939-09-01' THEN 'Pre WWII' ELSE 'Post WWII' END)

  FROM

  People

  WHERE

  (CASE WHEN People.Birthdate <= '1939-09-01' THEN 'Pre WWII' ELSE 'Post WWII' END) = 'Pre WWII'

  I have to do this to test for a derived type of logic that I have to test for and at the same time show. I always feel shotty when writing a query that uses this logic cause it seems like its doing the same calculation twice...

• mister.magoo

  SSC-Forever

  Points: 47174

  More actions

  November 18, 2010 at 4:45 pm

  #1252798

  I would say it does not calculate it twice.

  You can try it yourself by viewing the text or xml execution plan for the query- it will be obvious if it is there twice.

  Also, to make you feel warmer inside, try using cross apply for this.

  SELECT

  People.FirstName,

  People.LastName,

  WWII_Status = WWII.Status

  FROM

  People

  CROSS APPLY (CASE WHEN People.Birthdate <= '1939-09-01' THEN 'Pre WWII' ELSE 'Post WWII' END) AS WWII(Status)

  WHERE

  WWII.Status = 'Pre WWII'

  MM
  

  ---

  
  select geometry::STGeomFromWKB(0x0106000000020000000103000000010000000B0000001000000000000840000000000000003DD8CCCCCCCCCC0840000000000000003DD8CCCCCCCCCC08408014AE47E17AFC3F040000000000104000CDCCCCCCCCEC3F9C999999999913408014AE47E17AFC3F9C99999999991340000000000000003D0000000000001440000000000000003D000000000000144000000000000000400400000000001040000000000000F03F100000000000084000000000000000401000000000000840000000000000003D0103000000010000000B000000000000000000143D000000000000003D009E99999999B93F000000000000003D009E99999999B93F8014AE47E17AFC3F400000000000F03F00CDCCCCCCCCEC3FA06666666666FE3F8014AE47E17AFC3FA06666666666FE3F000000000000003D1800000000000040000000000000003D18000000000000400000000000000040400000000000F03F000000000000F03F000000000000143D0000000000000040000000000000143D000000000000003D, 0);

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