SQL Server Team-based Development - Red Gate Software

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Chapter 11: SQL Refactoring
The problem with the solution in Listing 11-10 is that it causes four reads of
TestTable for every row returned by the outer SELECT statement (one read per
column), with SQL Server storing the intermediate data in an internal Worktable
(a temporary table in memory or tempdb).
The refactoring is actually quite easy; we just have to think about this in a different way.
We can place a condition somewhere other than in the WHERE clause. Can you guess
where? If you said, "In the HAVING statement," you would be generally correct but, for this
case, you would be wrong. The correct answer would be "In the SELECT statement." We
must remember the good old CASE statement and think of a different way to aggregate
our data.
For the number column, the CASE statement evaluates to 1 when the condition is met
and 0 when it's not, so, to get the count of the rows that match the condition, we simply
have to sum over the returned values. The same logic applies to the other aggregates,
and means that we don't need the correlated subqueries in the SELECT list and we only
have to scan the table a single time to return all the data we need. Listing 11-11 shows the
refactored reporting query.
SELECT name ,
SUM(CASE WHEN number % 2 = 0 THEN 1
ELSE 0
END) AS CountOfEvenNumbers ,
SUM(CASE WHEN number % 2 = 0 THEN number
ELSE 0
END) AS SumOfEvenNumbers ,
SUM(CASE WHEN type = 'F' THEN 1
ELSE 0
END) AS CountOfTypeF ,
SUM(CASE WHEN name LIKE 'A%' THEN 1
ELSE 0
END) AS CountOfNamesStartingWithA
FROM #TestTable
GROUP BY name
ORDER BY name