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ASP FAQ Tutorials :: Databases :: Other Articles :: How do I use a SELECT list alias in the WHERE or GROUP BY clause?

How do I use a SELECT list alias in the WHERE or GROUP BY clause?

Many people expect to be able to do this: 
    FullName = LastName + ', ' + FirstName 
    FROM Employees 
    WHERE FullName = 'Bertrand, Aaron'
However, this results in the following: 
Server: Msg 207, Level 16, State 3, Line 2 
Invalid column name 'FullName'.
The SELECT list is interpreted and evaluated last, so there is no way to use an alias defined there in other clauses of the statement. (One exception is the ORDER BY clause, which is not a T-SQL clause, but rather a cursor operator.) 
Joe Celko says the following about the order of operations in a SELECT statement:
    Here is how a SELECT works in SQL ... at least in theory. Real products will optimize things when they can. 
    1. Start in the FROM clause and build a working table from all of the joins, unions, intersections, and whatever other table constructors are there. The <table expression> AS <correlation name> option allows you give a name to this working table which you then have to use for the rest of the containing query. 
    2. Go to the WHERE clause and remove rows that do not pass criteria; that is, that do not test to TRUE (reject UNKNOWN and FALSE). The WHERE clause is applied to the working in the FROM clause. 
    3. Go to the optional GROUP BY clause, make groups and reduce each group to a single row, replacing the original working table with the new grouped table. The rows of a grouped table must be group characteristics: (1) a grouping column (2) a statistic about the group (i.e. aggregate functions) (3) a function or (4) an expression made up of the those three items. 
    4. Go to the optional HAVING clause and apply it against the grouped working table; if there was no GROUP BY clause, treat the entire table as one group. 
    5. Go to the SELECT clause and construct the expressions in the list. This means that the scalar subqueries, function calls and expressions in the SELECT are done after all the other clauses are done. The AS operator can give a name to expressions in the SELECT list, too. These new names come into existence all at once, but after the WHERE clause has been executed; you cannot use them in the SELECT list or the WHERE cluase for that reason. 
    6. Nested query expressions follow the usual scoping rules you would expect from a block structured language like C, Pascal, Algol, etc. Namely, the innermost queries can reference columns and tables in the queries in which they are contained.
    This means that a SELECT cannot have more columns than a GROUP BY; but it certainly can have fewer columns.
What we can take from all of this, is that you have to repeat your alias expression; so, the above query becomes: 
    FullName = LastName + ', ' + FirstName 
    FROM Employees 
    WHERE LastName + ', ' + FirstName = 'Bertrand, Aaron'
Of course, this can be a pain for more complex formulae, right? Thankfully, there are some workarounds. One is to use a derived table, e.g. 
            FullName = LastName + ', ' + FirstName 
            FROM Employees 
    ) sub 
    WHERE sub.FullName = 'Bertrand, Aaron'
Another workaround is to create a view: 
CREATE VIEW dbo.EmployeesConcat 
        FullName = LastName + ', ' + FirstName 
        FROM Employees 
-- then your SQL statement becomes a little easier to swallow: 
    FROM EmployeesConcat 
    WHERE FullName = 'Bertrand, Aaron'

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Created: 10/23/2003 | Last Updated: 10/23/2003 | broken links | helpful | not helpful | statistics
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