Select statement always execute/select columns in order?

Does all the columns in select statement gets selected one after another as listed?

Declare 开发者_如何学JAVA@X, @Y

    SELECT 
    @X = ColumnA*.25 + ColumnB*2.5,
    @Y = ColumnA*.5 + ColumnC*1.33,
    TOTAL = @X + @Y
    FROM SomeTable

Is the above query safe to use? Will total always be selected after @X and @Y are calculated?

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You cannot mix column selection and variable assignments in one query.

If you select the total into a variable:

SELECT  @X = ColumnA*.25 + ColumnB*2.5,
        @Y = ColumnA*.5 + ColumnC*1.33,
        @TOTAL = @X + @Y
FROM    SomeTable

, then yes, @total will be assigned after @x and @y are calculated.

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If you intend to return a result set, I don't think this will work. It's better to write this in a portable way:

SELECT X, Y, X + Y AS TOTAL
  FROM (SELECT ColumnA*0.25 + ColumnB*2.5 AS X,
               ColumnA*0.5 + ColumnC*1.33 AS Y,
          FROM SomeTable) xxx

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put it in a Common Table Expression CTE

CTE = more maintainable/readable queries

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